White inkjet ink improved for dispersion stability

ABSTRACT

An inkjet printing method includes the steps of
         a) providing a white inkjet ink and at least one colour inkjet ink to an inkjet printer; and   b) jetting the white inkjet ink at a higher temperature than the colour inkjet ink onto an ink-receiver. The difference in jetting temperature between the white inkjet ink and the colour inkjet ink is at least 5° C. Inkjet ink sets and inkjet printers are also disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a 371 National Stage Application ofPCT/EP2008/063948, filed Oct. 16, 2008. This application claims thebenefit of U.S. Provisional Application No. 60/982,476, filed Oct. 25,2007, which is incorporated by reference herein in its entirety. Inaddition, this application claims the benefit of European ApplicationNo. 07119175.3, filed Oct. 24, 2007, which is also incorporated byreference herein in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to inkjet printing methods and inkjet inksets using white inkjet ink.

2. Description of the Related Art

In inkjet printing, tiny drops of ink fluid are projected directly ontoan ink-receiver surface without physical contact between the printingdevice and the ink-receiver. The printing device stores the printingdata electronically and controls a mechanism for ejecting the dropsimage-wise. Printing is accomplished by moving a print head across theink-receiver or vice versa or both.

When jetting the inkjet ink onto an ink-receiver, the ink typicallyincludes a liquid vehicle and one or more solids, such as dyes orpigments and polymeric binders. It will be readily understood that theoptimal composition of such ink is dependent on the printing method usedand on the nature of the ink-receiver to be printed. The inkcompositions can be roughly divided in:

-   -   water-based, the drying mechanism involving absorption,        penetration and evaporation;    -   solvent-based, the drying primarily involving evaporation;    -   oil-based, the drying involving absorption and penetration;    -   hot melt or phase change, in which the ink is liquid at the        ejection temperature but solid at room temperature and wherein        drying is replaced by solidification; and    -   UV-curable, in which drying is replaced by polymerization.

It should be clear that the first three types of ink compositions aremore suitable for a receiving medium that is more or less absorptive,whereas hot melt inks and UV-curable inks are usually printed onnon-absorbent ink-receivers.

White inkjet inks are generally used for so-called “surface printing” or“backing printing” to form a reflection image on a transparentsubstrate. In surface printing, a white background is formed on atransparent substrate using a white ink and further thereon, a colorimage is printed, whereafter the formed final image is viewed from theprinted face. In so-called backing printing, a color image is formed ona transparent substrate using color inks and then a white ink is appliedonto the color inks, and the final formed image is observed through thetransparent substrate. In a preferred embodiment the colour inkjet inkis jetted on partially cured white inkjet ink. If the white ink is onlypartially cured, an improved wettability of the colour ink on the whiteink layer is observed. Partially curing immobilizes the ink on thesubstrate surface. A quick test for determining if the white inkjet inkis partially cured can be done by rubbing a finger or a cloth across theprinted surface, whereby it is observed that ink can be smeared orsmudged on the surface.

Pigments with a high refractive index, such as titanium dioxide, areused in the white ink in order to obtain a sufficient opacity of theprinted layer. Sedimentation of these dense particles in a low viscosityfluid, such as an inkjet ink, is a real challenge for ink formulators.Problems of clogging of inkjet printhead nozzles and poor storagestability of the ink are direct consequences of sedimentation andaggregation of white pigments due to the difference in specific gravitybetween pigment particles and the liquid medium of the ink.

Various approaches have been used trying to overcome these problems. Oneapproach is to improve the dispersibility. EP 1388578 A (DAINIPPON INK)discloses an ultraviolet-curable ink composition for inkjet recordingincluding titanium oxide, a polymeric dispersant having a basicfunctional group, a photopolymerizable compound and aphotopolymerization initiator, the titanium oxide is surface-treatedwith silica and alumina and the weight of the silica, which coexistswith the titanium oxide, is larger than that of the alumina.

Another approach is designing particles which exhibit lesssedimentation. U.S. Pat. No. 4,880,465 (VIDEOJET) discloses anon-pigmented white inkjet ink including hollow microspheres containinga central void region filled with a liquid capable of diffusing throughthe walls of the microspheres and have an inside diameter from about 0.1to about 0.5 micron and an outside diameter from about 0.4 to about 1micron. Sedimentation is drastically reduced but the opacity of such aprinted white layer remains limited.

A third approach is the adaptation of the hardware involving anagitating device for reducing sedimentation, such as e.g. a stirrer inthe supply vessel of the white ink. However it is complex and costly toforesee an agitating device in the printhead and the tubing from thesupply vessel to the printhead.

EP 1803781 A1 (KONICA) discloses an active ray curable ink-jet ink setincluding colour inks and a white ink based on titanium dioxide.

WO 2007/035505 A1 (DU PONT) discloses an aqueous white ink including apolymerically dispersed titanium dioxide and a crosslinked polyurethanebinder.

EP 1818373 A2 (FUJIFILM) discloses an inkjet ink composition including:a white pigment; a polymerizable compound; and a polymerizationinitiator wherein the white pigment includes at least one of inorganichollow particles or inorganic-organic hybrid hollow particles.

It would be desirable to be able to print white layers of consistentquality on a wide variety of ink-receivers using a state-of-the-artinkjet printer not requiring any complex or costly adaptation of theprinter, wherein sedimentation problems of the pigment in the whiteinkjet ink are strongly reduced or eliminated.

SUMMARY OF THE INVENTION

Preferred embodiments of the present invention provide inkjet ink setsand inkjet printing methods improved for sedimentation problems of thewhite inkjet ink.

Further preferred embodiments of the invention will become apparent fromthe description hereinafter.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

TiO₂ is the white pigment of choice for a white inkjet ink primarilybecause its refractive index, which dictates its hiding power and thusopacity, is considerably higher than other white pigments (see Table 1).

TABLE 1 Refractive Density White Pigment Index (g/cm³) Rutile TiO₂ 2.734.3 Anatase TiO₂ 2.55 3.9 Zinc Oxide 2.02 5.6 White Lead 1.94-2.09 5.5Lithopone 1.84 4.3 Baryte 1.64 4.5 Calcium 1.63 2.8 carbonate

It was found that sedimentation problems in inkjet printing could bestrongly reduced by using higher concentrations of TiO₂, leading tohigher viscosity and reducing the viscosity of the ink in the inkjetprinter to a level that the inkjet ink became jettable by increasing thejetting temperature.

Preferred embodiments of the present invention have been realized aninkjet printing method as defined below.

Further advantages and embodiments of the present invention will becomeapparent from the following description.

Definitions

The term “colorant”, as used in disclosing the present invention meansdyes and pigments.

The term “dye”, as used in disclosing the present invention means acolorant having a solubility of 10 mg/L or more in the medium in whichit is applied and under the ambient conditions pertaining.

The term “pigment” is defined in DIN 55943, herein incorporated byreference, as a colouring agent that is practically insoluble in theapplication medium under the pertaining ambient conditions, hence havinga solubility of less than 10 mg/L therein.

The term “C.I.” is used in disclosing the present application as anabbreviation for Colour Index.

The term “UV” is used in disclosing the present invention as anabbreviation for ultraviolet radiation.

The term “ultraviolet radiation” as used in disclosing the presentinvention means electromagnetic radiation in the wavelength range of 100to 400 nanometers.

The term “wt %” is used in disclosing the present invention as anabbreviation for % by weight based upon the total weight of the inkunless otherwise specified.

The term “actinic radiation” as used in disclosing the present inventionmeans electromagnetic radiation capable of initiating photochemicalreactions.

The term “Norrish Type I initiator” as used in disclosing the presentinvention, means an initiator which cleaves after excitation, yieldingthe initiating radical immediately.

The term “Norrish Type II initiator” as used in disclosing the presentinvention, means an initiator which in its excited state forms freeradicals by hydrogen abstraction or electron extraction from a secondcompound that becomes the actual initiating free radical. The secondcompound is called co-initiator or polymerization synergist. Synergistsare compounds having a carbon atom with at least one hydrogen atom inthe α-position to a nitrogen atom.

The term “photo-acid generator” as used in disclosing the presentinvention means an initiator, which generates an acid or hemi-acid uponexposure to actinic radiation. A photo-acid generator is often alsocalled a cationic initiator.

The term “thermal initiator” as used in disclosing the present inventionmeans an initiator, which generates initiating species upon exposure toheat.

The term “alkyl” means all variants possible for each number of carbonatoms in the alkyl group i.e. for three carbon atoms: n-propyl andisopropyl; for four carbon atoms: n-butyl, isobutyl and tertiary-butyl;for five carbon atoms: n-pentyl, 1,1-dimethyl-propyl, 2,2-dimethylpropyland 2-methyl-butyl etc.

Inkjet Printing Methods

The inkjet printing method according to a preferred embodiment of thepresent invention includes the steps of:

-   a) providing a white inkjet ink and at least one colour inkjet ink    to an inkjet printer; and-   b) jetting the white inkjet ink at a higher temperature than the    colour inkjet ink onto an ink-receiver.

The printhead temperature control is used for reducing sedimentation ofa pigment with a refractive index greater than 1.60 in a white inkjetink.

The difference in jetting temperature between the white inkjet ink andthe colour inkjet ink is preferably at least 5° C., more preferably atleast 10° C. and most preferably at least 15° C.

The temperature of the white ink in the print head is preferably reducedwhen images are printed not requiring white ink or when the inkjetprinter is not used for more than 24 hours.

In a preferred embodiment, the temperature of the white ink in theprinthead is reduced while the printhead of the colour inkjet inkremains at its jetting temperature.

The inkjet printing method uses a colour inkjet ink set including atleast one colour ink and a white ink. The colour inkjet ink set includespreferably at least three colour inkjet inks and also a black inkjetink. Preferably the three colour inkjet inks are a cyan ink, a magentaink and a yellow ink. Red, green and blue inks are also sometimes usedas an inkjet ink set or added to a CMYK inkjet ink set.

The inkjet printing method may also use a so-called “multi-density”colour inkjet ink set, including colour inkjet inks of the same colourbut a different colour density. For example, the colour inkjet ink setmay include a “dark magenta” inkjet ink and a “light magenta” inkjetink. In a preferred embodiment, the multi-density inkjet ink setincludes dark and light colour inkjet inks for the colours magenta andcyan. Dark black and light black inks may also be present in a colourinkjet ink set. The inkjet ink set may include two, three, four or moreblack inks of different tone, a white ink and one or more colourlessliquids. The latter inkjet ink set is capable of providing very detailedblack and white images.

In one embodiment, the inkjet printing method uses a colour inkjet inkset including one or more colourless liquids. A colourless ink can, forexample, be used to enhance the glossiness of the printed image or as aprimer for improving the adhesion of the ink layers to the ink-receiver.

The adhesion may be influenced by using different polymerizablecompounds, surfactants, binders and/or organic solvents. For example, itis known that the adhesion of radiation curable inks is promoted onpolyvinyl chloride substrates when one or more monomers are used thatare suitable for the swelling of the PVC substrate and which areselected from the group consisting of tetrahydrofurfuryl acrylate,1,6-hexanediol diacrylate and N-vinyl caprolactam. However, adhesion onpolycarbonate substrates is promoted when one or more monomers are usedthat are suitable for the swelling of the polycarbonate substrate andwhich are selected from the group consisting of propoxylated neopentylglycol diacrylate, 2-phenoxylethyl acrylate, 2-(2-ethoxyethoxy)ethylacrylate and polyethyleneglycol diacrylate. With the radiation curableinkjet printing method according to a preferred embodiment of thepresent invention, it is not necessary to make a “best possible” mixtureof monomers suitable for both the swelling of polyvinyl chloridesubstrates and polycarbonate substrates. Instead, a dedicated monomermixture can be prepared consisting predominantly of monomers, forexample, for swelling polyvinyl chloride substrates if such a substrateis fed into the printer.

Inkjet Printer

Industrial inkjet printers generally include an ink supply system forsupplying ink to an inkjet print head. Inkjet print heads produce dropseither continuously or on demand. “Continuously” means that a continuousstream of ink drops is created, e.g. by pressurizing the ink supply. “Ondemand” differs from “continuous” in that ink drops are only ejectedfrom a print head by manipulation of a physical process to momentarilyovercome surface tension forces that keep the ink in the print head. Theink is held in a nozzle, forming a meniscus. The ink remains in placeunless some other force overcomes the surface tension forces that areinherent in the liquid. The most common practice is to suddenly raisethe pressure on the ink, ejecting it from the nozzle. One category ofdrop-on-demand inkjet print heads uses the physical phenomenon ofelectrostriction, a change in transducer dimension in response to anapplied electric field. Electrostriction is strongest in piezoelectricmaterials and hence these print heads are referred to as piezoelectricprint heads. The very small dimensional change of piezoelectric materialis harnessed over a large area to generate a volume change that is largeenough to squeeze out a drop of ink from a small chamber. Apiezoelectric print head includes a multitude of small ink chambers,arranged in an array, each having an individual nozzle and a percentageof transformable wall area to create the volume changes required toeject an ink drop from the nozzle, in accordance with electrostrictionprinciples.

In a preferred embodiment the inkjet printer is a drop-on-demand inkjetprinting system having piezoelectric print heads for delivering dropletsof a ink to an ink-receiver.

The inkjet ink is supplied to the ink ejecting chambers of a print headby an ink supply system that first conditions the ink in order to obtainsmooth operation of the inkjet print head. Conditioning includes, forexample, degassing of the ink and controlling the back-pressure at thenozzle.

It is known that the presence of air bubbles in the ink chamber of apiezoelectric print head often causes operational failure of the printhead. If air is present in the ink chamber, intended pressure changesresulting from piezoelectric deformation of part of the ink chamberwalls will be absorbed by the air, leaving the ink pressure unaffected.The surface tension force of the ink in the nozzle maintains themeniscus and no drops will be ejected from the ink chamber. At thefrequencies at which piezoelectric transducers in piezoelectric printhead are operated, i.e. in the kHz to MHz range, not only air bubblesbut also dissolved air in the ink can cause operation failure asdescribed above. In the prior art, concepts have been disclosed to avoidair bubbles in the ink chamber by creating an air trap upstream the inkchamber, i.e. prior to the ink entering the ink chamber. Solutions havebeen proposed in EP 714779 A (CANON) and U.S. Pat. No. 4,929,963 (HP) inthe form of air buffers or gas separators that allow air bubbles to riseand evacuate from the ink in an intermediate tank before the ink issupplied to the print head.

A second point of attention in ink supply systems is the pressure at thenozzle, which is critical to a well-tuned and good operating print head.Inkjet print heads operate best at a slightly negative nozzle pressureor back-pressure. In practice this is often achieved by maintaining aheight difference between the free ink surface in a vented ink supplytank and the meniscus in the nozzle. That is, the free ink surface inthe vented supply tank is maintained gravimetrically a couple ofcentimeters below the level of the meniscus in the nozzle. This heightdifference established a hydrostatic pressure difference to control theback-pressure at the nozzle. In reciprocating print head configurationsthe ink supply tank is located off axis, i.e. not scanning, becauseotherwise the lowered position of ink supply tank versus the print headwould interfere with the printing medium transport path. Flexible tubingis used to connect the off axis ink supply tank with the on axis printhead, as disclosed in for example U.S. Pat. No. 4,929,963 (HP). Duringacceleration and deceleration of the print head, pressure waves arecreated in the tubes that may significantly disturb the pressure balanceat the meniscus and may lead to weeping of the nozzle in the case of adecrease in negative pressure, or breaking of the meniscus in the caseof an increase in negative pressure and taking air into the ink channel.Many approaches have been proposed to control the back-pressure inreciprocating print head applications. A back-pressure regulationmechanism in the form of pressure buffers or dampers mounted togetherwith the print head on the reciprocating carriage are disclosed in EP1120257 A (SEIKO EPSON) and U.S. Pat. No. 6,485,137 (APRION DIGITAL).For accelerations and decelerations of the carriage above 1G theresponse time of these devices is insufficient. In EP 1142713 A (SEIKOEPSON) a vented subtank is used. The subtank serves as a local inkreservoir near the print head and is being filled intermittently from amain tank located off axis. The solution provides a better control ofthe nozzle back-pressure by maintaining a local hydrostatic pressuredifference between the free ink surface of the vented subtank and themeniscus.

The inkjet printer according to a preferred embodiment of the presentinvention includes a temperature control device to control thetemperature independently for a white inkjet ink printhead and one ormore colour inkjet ink printheads. In a more preferred embodiment, theinkjet printer also includes a back-pressure control device to controlthe back-pressure independently for a white inkjet ink printhead and oneor more colour inkjet ink printheads

Inkjet Ink-Receivers

The ink-receiver suitable for the inkjet printing method according to apreferred embodiment of the present invention is not restricted to anyspecific type and can be transparent, translucent or opaque. Theink-receiver may be coloured or metallized. It can be a temporarysubstrate, e.g. for transferring an image to another substrate afterprinting. Applications such as 3D-printing, direct printing on woodendoors or panels and ceramics are also included.

Aqueous inks are generally printed on absorbing ink-receivers. Solventbased inkjet inks and radiation curable inks can also be printed onink-receivers substantially non-absorbing for an aqueous solution. Forexample, standard paper is an absorbing ink-receiver. On the other hand,a resin-coated paper, e.g. polyethylene-coated paper orpolypropylene-coated paper, is usually substantially non-absorbing.

The ink-receiver may include a support with at least one ink-receivinglayer. The ink-receiving layer may consist of just one single layer, oralternatively it may be composed of two, three or more layers. Theink-receiving layer may contain one or more polymeric binders andoptionally fillers. The ink-receiving layer, and an optional auxiliarylayer, such as a backing layer for anti-curl and/or adhesive purposes,may further contain well-known conventional ingredients, such assurfactants serving as coating aids, cross-linking agents, plasticizers,cationic substances acting as mordant, light-stabilizers, pH adjusters,anti-static agents, biocides, lubricants, whitening agents and mattingagents.

The ink-receiving layer and the optional auxiliary layer(s) may becross-linked to a certain degree to provide such desired features aswaterfastness and non-blocking characteristics. The cross-linking isalso useful in providing abrasion resistance and resistance to theformation of fingerprints on the element as a result of handling.

Supports suitable for the ink-receiving layers are also suitableink-receivers for solvent based inkjet inks or radiation curable inksand include polymeric substrates such as cellulose acetate propionate,cellulose acetate butyrate, polyesters such as polyethyleneterephthalate (PET) and polyethylene naphthalate (PEN); orientedpolystyrene (OPS); oriented nylon (ONy); polypropylene (PP), orientedpolypropylene (OPP); polyvinyl chloride (PVC); and various polyamides,polycarbonates, polyimides, polyolefins, poly(vinylacetals), polyethersand polysulfonamides, opaque white polyesters and extrusion blends ofpolyethylene terephthalate and polypropylene. Acrylic resins, phenolresins, glass and metals may also be used as an ink-receiver. Othersuitable ink-receiver materials can be found in Modern Approaches toWettability: Theory and Applications. Edited by SCHRADER, Malcolm E., etal. New York: Plenum Press, 1992. ISBN 0306439859.

The ink-receiver may also incorporate mineral particles as fillers, suchas e.g. PET containing CaCO₃, PET containing TiO₂, amorphous PET (APET)and glycolized PET (PETG).

The ink-receiver may be provided with a self-adhesive backlayer.Examples of self-adhesive PVC ink-receivers include MPI™ vinyls fromAVERY-DENNISON, DIGITAL™ vinyls from METAMARK, MULTI-FIX™ digital whitevinyls from MULTI-FIX and GRAFIPRINT™ vinyls from GRAFITYP.

Polyester film substrates and especially polyethylene terephthalate arepreferred for certain applications, particularly types with excellentdimensional stability. When such a polyester substrate is used as theink-receiver, a subbing layer may be employed to improve the bonding ofthe jetted ink layer to the substrate, if it constitutes together withthe unsubbed substrate a substantially non-absorbing ink-receiver.Useful subbing layers for this purpose are well known in thephotographic art and include, for example, polymers of vinylidenechloride such as vinylidene chloride/acrylonitrile/acrylic acidterpolymers or vinylidene chloride/methyl acrylate/itaconic acidterpolymers. Stabilizers, leveling additives, matting agents, adjustingagents for physical film properties such as waxes, may also be added tothe subbing layer as required.

The ink-receiver may also be made from an inorganic material, such as ametal oxide or a metal (e.g. aluminium and steel).

Other suitable ink-receivers may be selected from the group consistingof cardboard, wood, composite boards, coated plastic, canvas, textile,glasses, plant fibre products, leather, magnetic materials and ceramics.

Inkjet Ink Sets

The colour inkjet ink set according to a preferred embodiment of thepresent invention includes a white ink containing a pigment with arefractive index greater than 1.60 and at least one colour inkjet inkwherein the viscosity of the white inkjet ink is at least 4 mPa·s, morepreferably at least 6 mPa·s, and most preferably at least 8 mPa·s,greater than the viscosity of the colour inkjet ink wherein theviscosity is measured at 40° C. with a Brookfield DV-II+Pro at 12rotations per minute.

A colour inkjet ink set according to a preferred embodiment of presentinvention includes preferably a white inkjet ink that is curable byradiation or electron beam. In a preferred embodiment the colour inksare also curable by radiation or electron beam.

The inkjet inks in a colour inkjet ink set according to a preferredembodiment of the present invention are preferably non-aqueous inkjetinks. In a non-aqueous inkjet ink the components are present in adispersion medium which is a non-aqueous liquid at jetting temperature.

The term “non-aqueous liquid” refers to a liquid carrier which shouldcontain no water. However sometimes a small amount, generally less than5 wt % of water based on the total weight of the ink, can be present.This water was not intentionally added but came into the formulation viaother components as a contamination, such as for example polar organicsolvents. Higher amounts of water than 5 wt % tend to make thenon-aqueous inkjet inks instable, preferably the water content is lessthan 1 wt % based on the total weight dispersion medium and mostpreferably no water at all is present.

The inkjet inks of a colour inkjet ink set according to a preferredembodiment of the present invention may further also contain at leastone surfactant.

The inkjet inks of a colour inkjet ink set according to a preferredembodiment of the present invention may contain at least one humectantto prevent the clogging of the nozzle, due to its ability to slow downthe evaporation rate of ink.

The pigmented inkjet inks according to a preferred embodiment of thepresent invention may contain at least one dispersion synergist. Amixture of dispersion synergists may be used to further improvedispersion stability

The inkjet inks of an ink set according to a preferred embodiment of thepresent invention is preferably an inkjet ink selected from the groupconsisting of an organic solvent based, an oil based and a curableinkjet ink. The curable inkjet ink is preferably radiation curable.

The viscosity of the inkjet ink is preferably smaller than 100 mPa·s at30° C. and at a shear rate of 100 s⁻¹. The viscosity of the inkjet inkis preferably smaller than 30 mPa·s, more preferably smaller than 15mPa·s, and most preferably between 2 and 10 mPa·s at a shear rate of 100s⁻¹ and a jetting temperature between 10 and 70° C.

The curable inkjet ink may contain as dispersion medium monomers,oligomers and/or prepolymers possessing different degrees offunctionality. A mixture including combinations of mono-, di-, tri-and/or higher functionality monomers, oligomers or prepolymers may beused. A catalyst called an initiator for initiating the polymerizationreaction may be included in the curable inkjet ink. The initiator can bea thermal initiator, but is preferably a photo-initiator. Thephoto-initiator requires less energy to activate than the monomers,oligomers and/or prepolymers to form the polymer. The photo-initiatorsuitable for use in the curable pigment dispersion may be a Norrish typeI initiator, a Norrish type II initiator or a photo-acid generator.

The curable inkjet inks of an ink set according to a preferredembodiment of the present invention may further also contain at leastone inhibitor.

In the most preferred embodiment, the ink set includes a cyan, magenta,yellow and black inkjet ink, i.e. a CMYK inkjet ink set.

A CMYK inkjet ink set may also be extended with one or more extra inkssuch as red, green, blue and orange to further enlarge the colour gamutof the image. The CMYK ink set may also be extended by the combinationof full density and light density inks of both colour inks and/or blackinks to improve the image quality by lowered graininess.

In another embodiment, the inkjet printing method uses a so-called“multi-density” inkjet ink set including colour inkjet inks of the samecolour but a different colour density. For example, the ink set mayinclude a “dark magenta” inkjet ink and a “light magenta” inkjet ink. Inanother preferred embodiment the multi-density inkjet ink set includesdark and light inkjet inks for the colours magenta and cyan. Dark blackand light black inks may also be present in an inkjet ink set. Theinkjet ink set may include two, three, four or more black inks ofdifferent tone, a white ink and one or more colourless liquids. Thelatter inkjet ink set is capable of providing very detailed black andwhite images.

White Inkjet Ink

The white inkjet ink includes a pigment with a refractive index greaterthan 1.60, preferably greater than 2.00, more preferably greater than2.50 and most preferably greater than 2.60. Suitable pigments are givenby Table 2. The white pigments may be employed singly or in combination.Preferably titanium dioxide is used for the pigment with a refractiveindex greater than 1.60.

TABLE 2 C.I. Number Chemical name CAS RN Pigment white 1 Lead hydroxide1319-46-6 carbonate Pigment white 3 Lead sulfate 7446-14-2 Pigment white4 Zinc oxide 1314-13-2 Pigment white 5 Lithopone 1345-05-7 Pigment white6 Titanium dioxide 13463-67-7 Pigment white 7 Zinc sulfide 1314-98-3Pigment white 10 Barium carbonate 513-77-9 Pigment white 11 Antimonytrioxide 1309-64-4 Pigment white 12 Zirconium oxide 1314-23-4 Pigmentwhite 14 Bismuth oxychloride 7787-59-9 Pigment white 17 Bismuthsubnitrate 1304-85-4 Pigment white 18 Calcium carbonate 471-34-1 Pigmentwhite 19 Kaolin 1332-58-7 Pigment white 21 Barium sulfate 7727-43-7Pigment white 24 Aluminum hydroxide 21645-51-2 Pigment white 25 Calciumsulfate 7778-18-9 Pigment white 27 Silicon dioxide 7631-86-9 Pigmentwhite 28 Calcium metasilicate 10101-39-0 Pigment white 32 Zinc phosphatecement 7779-90-0

Titanium oxide occurs in the crystalline forms of anatase type, rutiletype and brookite type. The anatase type has a relatively low densityand is easily ground into fine particles, while the rutile type has arelatively high refractive index, exhibiting a high covering power.Either one of these is usable in preferred embodiments of the invention.It is preferred to make the most possible use of characteristics and tomake selections according to the use thereof. The use of the anatasetype having a low density and a small particle size can achieve superiordispersion stability, ink storage stability and ejectability. At leasttwo different crystalline forms may be used in combination. The combineduse of the anatase type and the rutile type which exhibits a highcoloring power can reduce the total amount of titanium oxide, leading toimproved storage stability and ejection performance of ink.

For surface treatment of the titanium oxide, an aqueous treatment or agas phase treatment is applied, and an alumina-silica treating agent isusually employed. Untreated-, alumina treated- or alumina-silicatreated-titanium oxide are employable.

The numeric average particle diameter of the titanium oxide ispreferably from 150 to 500 nm, more preferably from 200 to 400 nm, andmost preferably from 230 to 350 nm. Sufficient hiding power cannot beobtained when the average diameter is less than 50 nm, and the storageability and the jet-out suitability of the ink tend to be degraded whenthe average diameter exceeds 500 nm.

The white inkjet ink is preferably curable by radiation or electronbeam. For improving adhesion on specific substrates, the white inkjetink preferably includes a polymerizable compound selected from the groupconsisting of isobornylacrylate, phenoxyethyl acrylate,tetrahydrofurfuryl acrylate, 2-(2-vinyloxyethoxy)ethyl(meth)acrylate andN-vinylcaprolactam.

The white inkjet ink preferably includes the white pigment in an amountof at least 26 wt %, more preferably 30 wt % and most preferably 40 wt %of white pigment based upon the total weight of the white inkjet ink.

Colour Inkjet Inks

The colour inkjet inks of a colour inkjet ink set according to apreferred embodiment of the present invention contain at least onecolorant. Colorants used in the inkjet inks may be pigments, dyes or acombination thereof. Organic and/or inorganic pigments may be used. Thecolour inkjet inks of the inkjet ink set according to a preferredembodiment of the present invention preferably contain a pigment ascolorant. If the colorant is not a self-dispersible pigment, the inkjetinks preferably also contain a dispersant, more preferably a polymericdispersant.

The radiation curable inkjet inks or solvent based inkjet inkspreferably contain pigments as colorants.

The pigments in the colour inkjet inks may be black, cyan, magenta,yellow, red, orange, violet, blue, green, brown, mixtures thereof, andthe like.

The colour pigment may be chosen from those disclosed by HERBST, Willy,et al. Industrial Organic Pigments, Production, Properties,Applications. 3rd edition. Wiley—VCH, 2004. ISBN 3527305769.

Particular preferred pigments are C.I. Pigment Yellow 1, 3, 10, 12, 13,14, 17, 55, 65, 73, 74, 75, 83, 93, 97, 109, 111, 120, 128, 138, 139,150, 151, 154, 155, 180,185 and 213.

Particular preferred pigments are C.I. Pigment Yellow 120, 151, 154,175, 180, 181 and 194.

The most preferred yellow pigments are C.I. Pigment Yellow 120, 139, 150155 and 213.

Particular preferred pigments are C.I. Pigment Red 17, 22, 23, 41, 48:1,48:2, 49:1, 49:2, 52:1, 57:1, 81:1, 81:3, 88, 112, 122, 144, 146, 149,169, 170, 175, 176, 184, 185, 188, 202, 206, 207, 210, 216, 221, 248,251, 254, 255, 264, 270 and 272. For manufacturing decorative laminates,the most preferred are C.I. Pigment Red 254 and C.I. Pigment Red 266.For other non-aqueous inkjet applications the most preferred pigmentsare C.I. Pigment Red 122 and C.I. Pigment Violet 19.

Particular preferred pigments are C.I. Pigment Violet 1, 2, 19, 23, 32,37 and 39.

Particular preferred pigments are C.I. Pigment Blue 15:1, 15:2, 15:3,15:4, 15:6, 16, 56, 61 and (bridged) aluminium phthalocyanine pigments.

Particular preferred pigments are C.I. Pigment Orange 5, 13, 16, 34, 40,43, 59, 66, 67, 69, 71 and 73.

Particular preferred pigments are C.I. Pigment Green 7 and 36.

Particular preferred pigments are C.I. Pigment Brown 6 and 7.

Suitable pigments include mixed crystals of the above particularpreferred pigments. A commercially available example is CINQUASIA™Magenta RT-355-D from Ciba Specialty Chemicals.

Carbon black is preferred as a pigment for the black inkjet ink.Suitable black pigment materials include carbon blacks such as PigmentBlack 7 (e.g. Carbon Black MAB™ from MITSUBISHI CHEMICAL), REGAL™ 400R,MOGUL™ L, ELFTEX™ 320 from CABOT Co., or Carbon Black FW18, SpecialBlack 250, Special Black 350, Special Black 550, PRINTEX™ 25, PRINTEX™35, PRINTEX™ 55, PRINTEX™ 90, PRINTEX™ 150T from DEGUSSA. Additionalexamples of suitable pigments are disclosed in U.S. Pat. No. 5,389,133(XEROX).

It is also possible to make mixtures of pigments in the colour inkjetinks. For some applications, a neutral black inkjet ink is preferred andcan be obtained, for example, by mixing a black pigment and a cyanpigment into the ink. The inkjet application may also require one ormore spot colours, for example for packaging inkjet printing or textileinkjet printing. Silver and gold are often desired colours for inkjetposter printing and point-of-sales displays.

Also non-organic pigments may be present in the colour inkjet inks.Particular preferred pigments are C.I. Pigment Metal 1, 2 and 3.Illustrative examples of the inorganic pigments include red iron oxide(III), cadmium red, ultramarine blue, prussian blue, chromium oxidegreen, cobalt green, amber, titanium black and synthetic iron black.

Generally pigments are stabilized in the dispersion medium by dispersingagents, such as polymeric dispersants or surfactants. However, thesurface of the pigments can be modified to obtain so-called“self-dispersible” or “self-dispersing” pigments, i.e. pigments that aredispersible in the dispersion medium without dispersants.

Pigment particles in inkjet ink should be sufficiently small to permitfree flow of the ink through the inkjet-printing device, especially atthe ejecting nozzles. It is also desirable to use small particles formaximum colour strength and to slow down sedimentation.

The numeric average pigment particle size is preferably between 0.050and 1 μm, more preferably between 0.070 and 0.300 μm and particularlypreferably between 0.080 and 0.200 μm. Most preferably, the numericaverage pigment particle size is no larger than 0.150 μm.

The pigment is preferably used in the pigment dispersion used forpreparing the inkjet inks in an amount of 10 to 40 wt %, preferably 15to 30 wt % based on the total weight of the pigment dispersion. In theinkjet ink the pigment is preferably used in an amount of 0.1 to 20 wt%, preferably 1 to 10 wt % based on the total weight of the inkjet ink.

Dyes suitable for the colour inkjet inks in the ink set according to apreferred embodiment of the present invention include direct dyes,acidic dyes, basic dyes and reactive dyes.

Suitable direct dyes for the colour inkjet inks include:

-   -   C.I. Direct Yellow 1, 4, 8, 11, 12, 24, 26, 27, 28, 33, 39, 44,        50, 58, 85, 86, 100, 110, 120, 132, 142, and 144    -   C.I. Direct Red 1, 2, 4, 9, 11, 134, 17, 20, 23, 24, 28, 31, 33,        37, 39, 44, 47, 48, 51, 62, 63, 75, 79, 80, 81, 83, 89, 90, 94,        95, 99, 220, 224, 227 and 343    -   C.I. Direct Blue 1, 2, 6, 8, 15, 22, 25, 71, 76, 78, 80, 86, 87,        90, 98, 106, 108, 120, 123, 163, 165, 192, 193, 194, 195, 196,        199, 200, 201, 202, 203, 207, 236, and 237    -   C.I. Direct Black 2, 3, 7, 17, 19, 22, 32, 38, 51, 56, 62, 71,        74, 75, 77, 105, 108, 112, 117, 154 and 195

Suitable acidic dyes for the colour inkjet inks include:

-   -   C.I. Acid Yellow 2, 3, 7, 17, 19, 23, 25, 20, 38, 42, 49, 59,        61, 72, and 99    -   C.I. Acid Orange 56 and 64    -   C.I. Acid Red 1, 8, 14, 18, 26, 32, 37, 42, 52, 57, 72, 74, 80,        87, 115, 119, 131, 133, 134, 143, 154, 186, 249, 254, and 256    -   C.I. Acid Violet 11, 34, and 75    -   C.I. Acid Blue 1, 7, 9, 29, 87, 126, 138, 171, 175, 183, 234,        236, and 249    -   C.I. Acid Green 9, 12, 19, 27, and 41    -   C.I. Acid Black 1, 2, 7, 24, 26, 48, 52, 58, 60, 94, 107, 109,        110, 119, 131, and 155

Suitable reactive dyes for the colour inkjet inks include:

-   -   C.I. Reactive Yellow 1, 2, 3, 14, 15, 17, 37, 42, 76, 95, 168,        and 175    -   C.I. Reactive Red 2, 6, 11, 21, 22, 23, 24, 33, 45, 111, 112,        114, 180, 218, 226, 228, and 235    -   C.I. Reactive Blue 7, 14, 15, 18, 19, 21, 25, 38, 49, 72, 77,        176, 203, 220, 230, and 235    -   C.I. Reactive Orange 5, 12, 13, 35, and 95    -   C.I. Reactive Brown 7, 11, 33, 37, and 46    -   C.I. Reactive Green 8 and 19    -   C.I. Reactive Violet 2, 4, 6, 8, 21, 22, and 25    -   C.I. Reactive Black 5, 8, 31, and 39

Suitable basic dyes for the colour inkjet inks include:

-   -   C.I. Basic Yellow 11, 14, 21, and 32    -   C.I. Basic Red 1, 2, 9, 12, and 13    -   C.I. Basic Violet 3, 7, and 14    -   C.I. Basic Blue 3, 9, 24, and 25

If the colour inkjet ink contains water, dyes can only manifest theideal colour in an appropriate range of pH value. Therefore, the inkjetink preferably further includes a pH adjuster.

Suitable pH adjusters include NaOH, KOH, NEt₃, NH₃, HCl, HNO₃, H₂SO₄ and(poly)alkanolamines such as triethanolamine and2-amino-2-methyl-1-propaniol. Preferred pH adjusters are NaOH and H₂SO₄.

The dyes are used in the colour inkjet inks in an amount of 0.1 to 30 wt%, preferably 1 to 20 wt % based on the total weight of the inkjet ink.

In a specific embodiment the colorant is a fluorescent colorant used tointroduce security features. Suitable examples of a fluorescent colorantinclude TINOPAL™ grades such as TINOPAL™ SFD, UVITEX™ grades such asUVITEX™ NFW and UVITEX™ OB, all available from CIBA SPECIALTY CHEMICALS;LEUKOPHOR™ grades from CLARIANT and BLANCOPHOR™ grades such asBLANCOPHOR™ REU and BLANCOPHOR™ BSU from BAYER.

Dispersants

The dispersant is preferably a polymeric dispersant. Typical polymericdispersants are copolymers of two monomers but may contain three, four,five or even more monomers. The properties of polymeric dispersantsdepend on both the nature of the monomers and their distribution in thepolymer. Suitable copolymeric dispersants have the following polymercompositions:

-   -   statistically polymerized monomers (e.g. monomers A and B        polymerized into ABBAABAB);    -   alternating polymerized monomers (e.g. monomers A and B        polymerized into ABABABAB);    -   gradient (tapered) polymerized monomers (e.g. monomers A and B        polymerized into AAABAABBABBB);    -   block copolymers (e.g. monomers A and B polymerized into        AAAAABBBBBB) wherein the block length of each of the blocks (2,        3, 4, 5 or even more) is important for the dispersion capability        of the polymeric dispersant;    -   graft copolymers (graft copolymers consist of a polymeric        backbone with polymeric side chains attached to the backbone);        and    -   mixed forms of these polymers, e.g. blocky gradient copolymers.

Polymeric dispersants may have different polymer architecture includinglinear, comb/branched, star, dendritic (including dendrimers andhyperbranched polymers). A general review on the architecture ofpolymers is given by ODIAN, George, Principles of Polymerization, 4thedition, Wiley-Interscience, 2004, p. 1-18.

Comb/branched polymers have side branches of linked monomer moleculesprotruding from various central branch points along the main polymerchain (at least 3 branch points).

Star polymers are branched polymers in which three or more eithersimilar or different linear homopolymers or copolymers are linkedtogether to a single core.

Dendritic polymers include the classes of dendrimers and hyperbranchedpolymers. In dendrimers, with well-defined mono-disperse structures, allbranch points are used (multi-step synthesis), while hyperbranchedpolymers have a plurality of branch points and multifunctional branchesthat lead to further branching with polymer growth (one-steppolymerization process).

Suitable polymeric dispersants may be prepared via addition orcondensation type polymerizations. Polymerization methods include thosedescribed by ODIAN, George, Principles of Polymerization, 4th edition,Wiley-Interscience, 2004, p. 39-606.

Addition polymerization methods include free radical polymerization(FRP) and controlled polymerization techniques. Suitable controlledradical polymerization methods include:

-   -   RAFT: reversible addition-fragmentation chain transfer;    -   ATRP: atom transfer radical polymerization    -   MADIX: reversible addition-fragmentation chain transfer process,        using a transfer active xanthate;    -   Catalytic chain transfer (e.g. using cobalt complexes);    -   Nitroxide (e.g. TEMPO) mediated polymerizations;

Other suitable controlled polymerization methods include:

-   -   GTP: group transfer polymerization;    -   Living cationic (ring-opening) polymerizations;    -   Anionic co-ordination insertion ring-opening polymerization; and    -   Living anionic (ring-opening) polymerization.

Reversible addition-fragmentation transfer (RAFT): controlledpolymerization occurs via rapid chain transfer between growing polymerradicals and dormant polymer chains. A review article on RAFT synthesisof dispersants with different polymeric geometry is given in QUINN J. F.et al., Facile Synthesis of comb, star, and graft polymers viareversible addition-fragmentation chain transfer (RAFT) polymerization,Journal of Polymer Science, Part A: Polymer Chemistry, Vol. 40,2956-2966, 2002.

Group transfer polymerization (GTP): the method of GTP used forsynthesis of AB block copolymers is disclosed by SPINELLI, Harry J, GTPand its use in water based pigment dispersants and emulsion stabilisers,Proc. of 20th Int. Conf. Org. Coat. Sci. Technol., New Platz, N.Y.,State Univ. N.Y., Inst. Mater. Sci. p. 511-518.

The synthesis of dendritic polymers is described in the literature. Thesynthesis of dendrimers in NEWCOME, G. R., et al. Dendritic Molecules:Concepts, Synthesis, Perspectives. VCH: WEINHEIM, 2001. Hyperbranchingpolymerization is described by BURCHARD, W. Solution properties ofbranched macromolecules. Advances in Polymer Science. 1999, vol. 143,no. II, p. 113-194. Hyperbranched materials can be obtained bypolyfunctional polycondensation as disclosed by FLORY, P. J. Molecularsize distribution in three-dimensional polymers. VI. Branched polymercontaining A-R-Bf-1-type units. Journal of the American ChemicalSociety. 1952, vol. 74, p. 2718-1723.

Living cationic polymerizations is e.g. used for the synthesis ofpolyvinyl ethers as disclosed in WO 2005/012444 (CANON), US 20050197424(CANON) and US 20050176846 (CANON). Anionic co-ordination ring-openingpolymerization is e.g. used for the synthesis of polyesters based onlactones. Living anionic ring-opening polymerization is e.g. used forthe synthesis of polyethylene oxide macromonomers.

Free radical Polymerization (FRP) proceeds via a chain mechanism, whichbasically consists of four different types of reactions involving freeradicals: (1) radical generation from non-radical species (initiation),(2) radical addition to a substituted alkene (propagation), (3) atomtransfer and atom abstraction reactions (chain transfer and terminationby disproportionation), and (4) radical-radical recombination reactions(termination by combination).

Polymeric dispersants having several of the above polymer compositionsare disclosed in U.S. Pat. No. 6,022,908 (HP), U.S. Pat. No. 5,302,197(DU PONT) and U.S. Pat. No. 6,528,557 (XEROX).

Suitable statistical copolymeric dispersants are disclosed in U.S. Pat.No. 5,648,405 (DU PONT), U.S. Pat. No. 6,245,832 (FUJI XEROX), U.S. Pat.No. 6,262,207 (3M), US 20050004262 (KAO) and U.S. Pat. No. 6,852,777(KAO).

Suitable alternating copolymeric dispersants are described in US20030017271 (AKZO NOBEL).

Suitable block copolymeric dispersants have been described in numerouspatents, especially block copolymeric dispersants containing hydrophobicand hydrophilic blocks. For example, U.S. Pat. No. 5,859,113 (DU PONT)discloses AB block copolymers, U.S. Pat. No. 6,413,306 (DU PONT)discloses ABC block copolymers.

Suitable graft copolymeric dispersants are described in CA 2157361 (DUPONT) (hydrophobic polymeric backbone and hydrophilic side chains);other graft copolymeric dispersants are disclosed in U.S. Pat. No.6,652,634 (LEXMARK), U.S. Pat. No. 6,521,715 (DU PONT).

Suitable branched copolymeric dispersants are described U.S. Pat. No.6,005,023 (DU PONT), U.S. Pat. No. 6,031,019 (KAO), U.S. Pat. No.6,127,453 (KODAK).

Suitable dendritic copolymeric dispersants are described in e.g. U.S.Pat. No. 6,518,370 (3M), U.S. Pat. No. 6,258,896 (3M), US 2004102541(LEXMARK), U.S. Pat. No. 6,649,138 (QUANTUM DOT), US 2002256230 (BASF),EP 1351759 A (EFKA ADDITIVES) and EP 1295919 A (KODAK).

Suitable designs of polymeric dispersants for inkjet inks are disclosedin SPINELLI, Harry J., Polymeric Dispersants in Inkjet technology,Advanced Materials, 1998, Vol. 10, no. 15, p. 1215-1218.

The monomers and/or oligomers used to prepare the polymeric dispersantcan be any monomer and/or oligomer found in the Polymer Handbook Vol.1+2, 4th edition, edited by J. BRANDRUP et al., Wiley-Interscience,1999.

Polymers useful as pigment dispersants include naturally occurringpolymers, and specific examples thereof include: proteins, such as glue,gelatine, casein, and albumin; naturally occurring rubbers, such as gumarabic and tragacanth; glucosides such as saponin; alginic acid andalginic acid derivatives, such as propylene glycol alginate; andcellulose derivatives, such as methyl cellulose, carboxymethyl celluloseand ethylhydroxy cellulose; wool and silk, and synthetic polymers.

Suitable examples of monomers for synthesising polymeric dispersantsinclude: acrylic acid, methacrylic acid, maleic acid (or there salts),maleic anhydride, alkyl(meth)acrylates (linear, branched and cycloalkyl)such as methyl(meth)acrylate, n-butyl(meth)acrylate,tert-butyl(meth)acrylate, cyclohexyl(meth)acrylate, and2-ethylhexyl(meth)acrylate; aryl(meth)acrylates such asbenzyl(meth)acrylate, and phenyl(meth)acrylate;hydroxyalkyl(meth)acrylates such as hydroxyethyl(meth)acrylate, andhydroxypropyl(meth)acrylate; (meth)acrylates with other types offunctionalities (e.g. oxiranes, amino, fluoro, polyethylene oxide,phosphate substituted) such as glycidyl(meth)acrylate,dimethylaminoethyl(meth)acrylate, trifluoroethyl acrylate,methoxypolyethyleneglycol(meth)acrylate, andtripropyleneglycol(meth)acrylate phosphate; allyl derivatives such asallyl glycidyl ether; styrenics such as styrene, 4-methylstyrene,4-hydroxystyrene, 4-acetostyrene, and styrene sulfonic acid;(meth)acrylonitrile; (meth)acrylamides (including N-mono andN,N-disubstituted) such as N-benzyl(meth)acrylamide; maleimides such asN-phenyl maleimide; vinyl derivatives such as vinyl alcohol,vinylcaprolactam, vinylpyrrolidone, vinylimidazole, vinylnapthalene, andvinyl halides; vinylethers such as vinylmethyl ether; vinylesters ofcarboxylic acids such as vinylacetate, vinylbutyrate, and vinylbenzoate. Typical condensation type polymers include polyurethanes,polyamides, polycarbonates, polyethers, polyureas, polyimines,polyimides, polyketones, polyester, polysiloxane, phenol-formaldehyde,urea-formaldehyde, melamine-formaldehyde, polysulfide, polyacetal orcombinations thereof.

Suitable copolymeric dispersants are acrylic acid/acrylonitrilecopolymer, vinyl acetate/acrylic ester copolymer, acrylic acid/acrylicester copolymer, styrene/acrylic acid copolymer, styrene/methacrylicacid copolymer, styrene/methacrylic acid/acrylic ester copolymer,styrene/α-methylstyrene/acrylic acid copolymer,styrene/α-methylstyrene/acrylic acid/acrylic ester copolymer,styrene/maleic acid copolymer, styrene/maleic anhydride copolymer,vinylnaphthalene/acrylic acid copolymer, vinylnapthalene/maleic acidcopolymer, vinyl acetate/ethylene copolymer, vinyl acetate/fattyacid/ethylene copolymer, vinyl acetate/maleic ester copolymer, vinylacetate/crotonic acid copolymer, vinyl acetate/acrylic acid copolymer.

Suitable chemistries of copolymeric dispersants also include:

-   -   Copolymers which are the product of a condensation process of        poly(ethylene imine) with a carboxylic acid terminated polyester        (made by addition polymerization); and    -   Copolymers which are the product of a reaction of a        multifunctional isocyanate with:        -   a compound monosubstituted with a group that is capable of            reacting with an isocyanate, e.g. polyester;        -   a compound containing two groups capable of reacting with an            isocyanate (cross-linker); and/or        -   a compound with at least one basic ring nitrogen and a group            that is capable of reacting with an isocyanate group.

A detailed list of suitable polymeric dispersants is disclosed by MCCUTCHEON, Functional Materials, North American Edition, Glen Rock, N.J.:Manufacturing Confectioner Publishing Co., 1990, p. 110-129.

Suitable pigment stabilisers are also disclosed in DE 19636382 (BAYER),U.S. Pat. No. 5,720,802 (XEROX), U.S. Pat. No. 5,713,993 (DU PONT), WO96/12772 (XAAR) and U.S. Pat. No. 5,085,689 (BASF).

One polymeric dispersant or a mixture of two or more polymericdispersants may be present to improve the dispersion stability further.Sometimes surfactants can also be used as pigment dispersants, thus acombination of a polymeric dispersant with a surfactant is alsopossible.

The polymeric dispersant can be non-ionic, anionic or cationic innature; salts of the ionic dispersants can also be used.

The polymeric dispersant has preferably a polymerization degree DPbetween 5 and 1,000, more preferably between 10 and 500 and mostpreferably between 10 and 100.

The polymeric dispersant has preferably a number average molecularweight Mn between 500 and 30,000, more preferably between 1,500 and10,000.

The polymeric dispersant has preferably a weight average molecularweight Mw smaller than 100,000, more preferably smaller than 50,000 andmost preferably smaller than 30,000.

The polymeric dispersant has preferably a polymeric dispersity PDsmaller than 2, more preferably smaller than 1.75 and most preferablysmaller than 1.5.

Commercial examples of polymeric dispersants are the following:

-   -   DISPERBYK™ dispersants available from BYK CHEMIE GMBH;    -   SOLSPERSE™ dispersants available from NOVEON;    -   TEGO™ DISPERS™ dispersants from DEGUSSA;    -   EDAPLAN™ dispersants from MUNZING CHEMIE;    -   ETHACRYL™ dispersants from LYONDELL;    -   GANEX™ dispersants from ISP;    -   DISPEX™ and EFKA™ dispersants from CIBA SPECIALTY CHEMICALS INC;    -   DISPONER™ dispersants from DEUCHEM; and    -   JONCRYL™ dispersants from JOHNSON POLYMER.

Particularly preferred polymeric dispersants include SOLSPERSE™dispersants from NOVEON, EFKA™ dispersants from CIBA SPECIALTY CHEMICALSINC and DISPERBYK™ dispersants from BYK CHEMIE GMBH.

Particularly preferred dispersants for UV-curable pigmented dispersionsare SOLSPERSE™ 32000, 35000 and 39000 dispersants from NOVEON.

Particularly preferred dispersants for oil based pigmented dispersionsare SOLSPERSE™ 11000, 11200, 13940, 16000, 17000 and 19000 from NOVEON.

The polymeric dispersant is preferably used in an amount of 2 to 600 wt%, more preferably 5 to 200 wt % based on the weight of the pigment.

Dispersion Synergists

The dispersion synergist usually consists of an anionic part and acationic part. The anionic part of the dispersion synergist exhibiting acertain molecular similarity with the colour pigment and the cationicpart of the dispersion synergist consists of one or more protons and/orcations to compensate the charge of the anionic part of the dispersionsynergist.

The synergist is preferably added in a smaller amount than the polymericdispersant(s). The ratio of polymeric dispersant/dispersion synergistdepends upon the pigment and should be determined experimentally.Typically the ratio wt % polymeric dispersant/wt % dispersion synergistis selected between 2:1 to 100:1, preferably between 2:1 and 20:1.

Suitable dispersion synergists that are commercially available includeSOLSPERSE™ 5000 and SOLSPERSE™ 22000 from NOVEON.

Particular preferred pigments for the magenta ink used are adiketopyrrolo-pyrrole pigment or a quinacridone pigment. Suitabledispersion synergists include those disclosed in EP 1790698 A (AGFAGRAPHICS) and EP 1790695 A (AGFA GRAPHICS).

In dispersing C.I. Pigment Blue 15:3, the use of a sulfonatedCu-phthalocyanine dispersion synergist, e.g. SOLSPERSE™ 5000 from NOVEONis preferred. Suitable dispersion synergists for yellow inkjet inksinclude those disclosed in EP 1790697 A (AGFA GRAPHICS).

Dispersion Media

In one embodiment the dispersion medium consists of organic solvent(s).Suitable organic solvents include alcohols, ketones, esters, ethers,glycols and polyglycols and derivatives thereof, lactones, N-containingsolvents such as amides. Preferably mixtures of one or more of thesesolvents are used.

Examples of suitable alcohols include methyl alcohol, ethyl alcohol,n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, heptyl alcohol,octyl alcohol, cyclohexyl alcohol, benzyl alcohol, phenylethyl alcohol,phenylpropyl alcohol, furfuryl alcohol, anise alcohol andfluoroalcohols.

Examples of suitable ketones include acetone, methyl ethyl ketone,methyl n-propyl ketone, methyl isopropyl ketone, methyl n-butyl ketone,methyl isobutyl ketone, methyl n-amyl ketone, methyl isoamyl ketone,diethyl ketone, ethyl n-propyl ketone, ethyl isopropyl ketone, ethyln-butyl ketone, ethyl isobutyl ketone, di-n-propyl ketone, diisobutylketone, cyclohexanone, methylcyclohexanone and isophorone,2,4-pentanedione and hexafluoroacetone.

Examples of suitable esters include methyl acetate, ethyl acetate,n-propyl acetate, isopropyl acetate, n-butyl acetate, isobutyl acetate,hexyl acetate, octyl acetate, benzyl acetate, phenoxyethyl acetate,ethyl phenyl acetate, methyl lactate, ethyl lactate, propyl lactate,butyl lactate; methyl propionate, ethyl propionate, benzyl propionate,ethylene carbonate, propylene carbonate, amyl acetate, ethyl benzoate,butyl benzoate, butyl laurate, isopropyl myristate, isopropyl palmirate,triethyl phosphate, tributyl phosphate, diethyl phthalate, dibutylphthalate, diethyl malonate, dipropyl malonate, diethyl succinate,dibutyl succinate, diethyl glutarate, diethyl adipate, dibutyl adipateand diethyl sebacate.

Examples of suitable ethers include butyl phenyl ether, benzyl ethylether, hexyl ether, diethyl ether, dipropyl ether, tetrahydrofuran anddioxane.

Examples of suitable glycols and polyglycols include ethylene glycol,diethylene glycol, triethylene glycol, propylene glycol, dipropyleneglycol and tripropylene glycol.

Examples of suitable glycol and polyglycol derivatives include etherssuch as alkylene glycol mono alkyl ethers, alkylene glycol dialkylethers, polyalkylene glycol monoalkyl ethers, polyalkylene glycoldialkyl ethers and esters of the preceding glycol ethers such as acetateand propionate esters, in case of dialkyl ethers only one ether function(resulting in mixed ether/ester) or both ether functions can beesterized (resulting in dialkyl ester).

Examples of suitable alkylene glycol mono alkyl ethers include ethyleneglycol mono methyl ether, ethylene glycol mono ethyl ether, ethyleneglycol mono propyl ether, ethylene glycol mono butyl ether, ethyleneglycol mono hexyl ether, ethylene glycol mono 2-ethyl-hexyl ether,ethylene glycol mono phenyl ether, propylene glycol mono methyl ether,propylene glycol mono ethyl ether, propylene glycol mono n-propyl ether,propylene glycol mono n-butyl ether, propylene glycol mono iso-butylether, propylene glycol mono t-butyl ether and propylene glycol monophenyl ether.

Examples of suitable alkylene glycol dialkyl ethers include ethyleneglycol dimethyl ether, ethylene glycol diethyl ether, ethylene glycolmethyl ethyl ether, ethylene glycol dibutyl ether, propylene glycoldimethyl ether, propylene glycol diethyl ether and propylene glycoldibutyl ether.

Examples of suitable polyalkylene glycol mono alkyl ethers includediethylene glycol mono methyl ether, diethylene glycol mono ethyl ether,diethylene glycol mono-n-propyl ether, diethylene glycol mono n-butylether, diethylene glycol mono hexyl ether, triethylene glycol monomethyl ether, triethylene mono ethyl ether, triethylene glycol monobutyl ether, dipropylene mono methyl ether, dipropylene glycol monoethyl ether, dipropylene glycol n-propyl ether, dipropylene glycol monon-butyl ether, dipropylene mono t-butyl ether, tripropylene glycol monomethyl ether, tripropylene glycol mono ethyl ether, tripropylene glycolmono n-propyl ether and tripropylene glycol mono n-butyl ether.

Examples of suitable polyalkylene glycol dialkyl ethers includediethylene glycol dimethyl ether, triethylene glycol dimethyl ether,tetraethylene glycol dimethyl ether, diethylene glycol diethyl ether,triethylene glycol diethyl ether, tetraethylene glycol diethyl ether,diethylene glycol methyl ethyl ether, triethylene glycol methyl ethylether, tetraethylene glycol methyl ethyl ether, diethylene glycoldi-n-propyl ether, diethylene glycol di-iso-propyl ether, dipropyleneglycol dimethyl ether, dipropylene glycol diethyl ether, dipropylene din-propyl ether, dipropylene di t-butyl ether, tripropylene glycoldimethyl ether and tripropylene glycol diethyl ether.

Examples of suitable glycol esters include ethylene glycol monomethylether acetate, ethylene glycol monoethyl ether acetate, ethylene glycolmonopropyl ether acetate, ethylene glycol monobutyl ether acetate,diethylene glycol monoethyl ether acetate, diethylene glycol monobutylether acetate, propylene glycol monomethyl ether acetate, propyleneglycol monoethyl ether acetate, dipropylene glycol monomethyl etheracetate and propylene glycol monomethyl ether propionate.

Preferred solvents for use in pigment dispersions and inkjet inks areone or more polyalkyleneglycol dialkylethers represented by the formula(PAG)

wherein,

-   R₁ and R₂ are each independently selected from an alkyl group having    1 to 4 carbon atoms;-   Y represents an ethylene group and/or a propylene group; wherein n    is an integer selected from 4 to 20. Preferably a mixture of two or    more polyalkyleneglycol dialkylethers represented by the formula    (PAG).

The alkyl groups R₁ and R₂ of the polyalkyleneglycol dialkylethersaccording to Formula (PAG) preferably represent methyl and/or ethyl.Most preferably the alkyl groups R₁ and R₂ are both methyl groups.

In a preferred embodiment the polyalkyleneglycol dialkylethers accordingto Formula (PAG) are polyethylene glycol dialkylethers.

In another preferred embodiment, a mixture of 2, 3, 4 or morepolyalkyleneglycol dialkylethers, more preferably polyethylene glycoldialkylethers are present in the pigment dispersion or inkjet ink.

Suitable mixtures of polyalkyleneglycol dialkylethers for the pigmentdispersions include mixtures of polyethylene glycol dimethyl ethershaving a molecular weight of at least 200, such as Polyglycol DME 200™,Polyglycol DME 250™ and Polyglycol DME 500™ from CLARIANT. Thepolyalkyleneglycol dialkylethers used in non-aqueous inkjet inks havepreferably an average molecular weight between 200 and 800, and morepreferably no polyalkyleneglycol dialkylethers with a molecular weightof more than 800 are present. The mixture of polyalkyleneglycoldialkylethers is preferably a homogeneous liquid mixture at roomtemperature.

Suitable commercial glycol ether solvents include CELLOSOLVE™ solventsand CARBITOL™ solvents from UNION CARBIDE, EKTASOLVE™ solvents fromEASTMAN, DOWANOL™ solvents from DOW, OXITOLL™ solvents, Dioxitoll™solvents, PROXITOLL™ solvents and DIPROXITOLL™ solvents from SHELLCHEMICAL and ARCOSOLV™ solvents from LYONDELL.

Lactones are compounds having a ring structure formed by ester bonds andcan be of the γ-lactone (5-membered ring structure), δ-lactone(6-membered ring structure) or ε-lactone (7-membered ring structure)types. Suitable examples of lactones include γ-butyrolactone,γ-valerolactone, γ-hexalactone, γ-heptalactone, γ-octalactone,γ-nonalactone, γ-decalactone, γ-undecalactone, δ-valerolactone,δ-hexalactone, δ-heptalactone, δ-octalactone, δ-nonalactone,δ-decalactone, δ-undecalactone and ε-caprolactone.

Suitable examples of N-containing organic solvents include2-pyrrolidone, N-methylpyrrolidone, N-ethyl-2-pyrrolidone,N-octyl-2-pyrrolidone, N-dodecyl-2-pyrrolidone, N,N-dimethylacetamid,N,N-dimethylformamid, acetonitril and N,N-dimethyldodecanamide.

In another embodiment the dispersion medium includes oil types ofliquids, alone or in combination with organic solvent(s). Suitableorganic solvents include alcohols, ketones, esters, ethers, glycols andpolyglycols and derivatives thereof, lactones, N-containing solventssuch as amides, higher fatty acid ester and mixtures of one or more ofthe solvents as described above for solvent based dispersion media.

The amount of polar solvent is preferably lower than the amount of oil.The organic solvent has preferably a high boiling point, preferablyabove 200° C. Examples of suitable combinations are disclosed by GB2303376 (FUJITSU ISOTEC) especially for the use of oleyl alcohol and EP1157070 A (MARCONI DATA SYSTEMS) for the combination of oil and volatileorganic solvent.

Suitable oils include saturated hydrocarbons and unsaturatedhydrocarbons, aromatic oils, paraffinic oils, extracted paraffinic oils,napthenic oils, extracted napthenic oils, hydrotreated light or heavyoils, vegetable oils, white oils, petroleum naphtha oils,halogen-substituted hydrocarbons, silicones and derivatives and mixturesthereof.

Hydrocarbons may be selected from straight chain or branched chainaliphatic hydrocarbons, alicyclic hydrocarbons and aromatichydrocarbons. Examples of hydrocarbons are saturated hydrocarbons suchas n-hexane, isohexane, n-nonane, isononane, dodecane and isododecane;unsaturated hydrocarbons such as 1-hexene, 1-heptene and 1-octene;cyclic saturated hydrocarbons such as cyclohexane, cycloheptane,cyclooctane, cyclodecane and decalin; cyclic unsaturated hydrocarbonssuch as cyclohexene, cycloheptene, cyclooctene,1,3,5,7-cyclooctatetraene; and cyclododecene; and aromatic hydrocarbonssuch as benzene, toluene, xylene, naphthalene, phenanthrene, anthraceneand derivatives thereof. In literature the term paraffinic oil is oftenused. Suitable Paraffinic oils can be normal paraffin type (octane andhigher alkanes), isoparaffins (isooctane and higher iso-alkanes) andcycloparaffins (cyclooctane and higher cycloalkanes) and mixtures ofparaffin oils. The term “liquid paraffin” is often used to refer to amixture of mainly including three components of a normal paraffin, anisoparaffin and a monocyclic paraffin, which is obtained by highlyrefining a relatively volatile lubricating oil fraction through asulphuric-acid washing or the like, as described in U.S. Pat. No.6,730,153 (SAKATA INX). Suitable hydrocarbons are also described asde-aromatized petroleum distillates.

Suitable examples of halogenated hydrocarbons include methylenedichloride, chloroform, tetrachloromethane and methyl chloroform. Othersuitable examples of halogen-substituted hydrocarbons includeperfluoro-alkanes, fluorine-based inert liquids and fluorocarboniodides.

Suitable examples of silicone oils include dialkyl polysiloxane (e.g.,hexamethyl disiloxane, tetramethyl disiloxane, octamethyl trisiloxane,hexamethyl trisiloxane, heptamethyl trisiloxane, decamethyltetrasiloxane, trifluoropropyl heptamethyl trisiloxane, diethyltetramethyl disiloxane), cyclic dialkyl polysiloxane (e.g., hexamethylcyclotrisiloxane, octamethyl cyclotetrasiloxane, tetramethylcyclotetrasiloxane, tetra(trifluoropropyl)tetramethylcyclotetrasiloxane), and methylphenyl silicone oil.

White oil is a term used for white mineral oils, which are highlyrefined mineral oils that consist of saturated aliphatic and alicyclicnon-polar hydrocarbons. White oils are hydrophobic, colourless,tasteless, odourless, and do not change colour over time.

Vegetable oils include semi-drying oils such as soybean oil, cotton seedoil, sunflower oil, rape seed oil, mustard oil, sesame oil and corn oil;non-drying oils such as olive oil, peanut oil and tsubaki oil; anddrying oils such as linseed oil and safflower oil, wherein thesevegetable oils can be used alone or as a mixture thereof.

Examples of other suitable oils include petroleum oils, non-drying oilsand semi-drying oils.

Commercially available suitable oils include the aliphatic hydrocarbonstypes such as the ISOPAR™ range (isoparaffins) and Varsol/Naphtha rangefrom EXXON CHEMICAL, the SOLTROL™ range and hydrocarbons from CHEVRONPHILLIPS CHEMICAL, and the SHELLSOL™ range from SHELL CHEMICALS.

Suitable commercial normal paraffins include the NORPAR™ range fromEXXON MOBIL CHEMICAL.

Suitable commercial napthenic hydrocarbons include the NAPPAR™ rangefrom EXXON MOBIL CHEMICAL.

Suitable commercial de-aromatized petroleum distillates include theEXXSOL™ D types from EXXON MOBIL CHEMICAL.

Suitable commercial fluoro-substituted hydrocarbons includefluorocarbons from DAIKIN INDUSTRIES LTD, Chemical Division.

Suitable commercial silicone oils include the silicone fluid ranges fromSHIN-ETSU CHEMICAL, Silicone Division.

Suitable commercial white oils include WITCO™ white oils from CROMPTONCORPORATION.

If the non-aqueous pigment dispersion is a curable pigment dispersion,the dispersion medium includes one or more monomers and/or oligomers toobtain a liquid dispersion medium. Sometimes, it can be advantageous toadd a small amount of an organic solvent to improve the dissolution ofthe dispersant. The content of organic solvent should be lower than 20wt % based on the total weight of the inkjet ink. In other cases, it canbe advantageous to add a small amount of water, for example, to improvethe spreading of the inkjet ink on a hydrophilic surface, but preferablythe inkjet ink contains no water.

Preferred organic solvents include alcohols, aromatic hydrocarbons,ketones, esters, aliphatic hydrocarbons, higher fatty acids, carbitols,cellosolves, higher fatty acid esters. Suitable alcohols includemethanol, ethanol, propanol and 1-butanol, 1-pentanol, 2-butanol,t.-butanol. Suitable aromatic hydrocarbons include toluene, and xylene.Suitable ketones include methyl ethyl ketone, methyl isobutyl ketone,2,4-pentanedione and hexafluoroacetone. Also glycol, glycolethers,N-methylpyrrolidone, N,N-dimethylacetamid, N,N-dimethylformamid may beused.

In the case of a curable inkjet ink, the dispersion medium preferablyconsists of monomers and/or oligomers.

Monomers and Oligomers

Any monomer or oligomer may be used as curable compound for the curableinkjet ink. A combination of monomers, oligomers and/or prepolymers mayalso be used. The monomers, oligomers and/or prepolymers may possessdifferent degrees of functionality, and a mixture including combinationsof mono-, di-, tri-and higher functionality monomers, oligomers and/orprepolymers may be used. The viscosity of the inkjet ink can be adjustedby varying the ratio between the monomers and oligomers.

Any method of conventional radical polymerization, photo-curing systemusing photo acid or photo base generator, or photo induction alternatingcopolymerization may be employed. In general, radical polymerization andcationic polymerization are preferred, and photo induction alternatingcopolymerization needing no initiator may also be employed. Furthermore,a hybrid system of combinations of these systems is also effective.

Cationic polymerization is superior in effectiveness due to lack ofinhibition of the polymerization by oxygen, however it is expensive andslow, especially under conditions of high relative humidity. If cationicpolymerization is used, it is preferred to use an epoxy compoundtogether with an oxetane compound to increase the rate ofpolymerization. Radical polymerization is the preferred polymerizationprocess.

Any polymerizable compound commonly known in the art may be employed.Particularly preferred for use as a radiation curable compound in theradiation curable inkjet ink are monofunctional and/or polyfunctionalacrylate monomers, oligomers or prepolymers, such as isoamyl acrylate,stearyl acrylate, lauryl acrylate, octyl acrylate, decyl acrylate,isoamylstyl acrylate, isostearyl acrylate, 2-ethylhexyl-diglycolacrylate, 2-hydroxybutyl acrylate, 2-acryloyloxyethylhexahydrophthalicacid, butoxyethyl acrylate, ethoxydiethylene glycol acrylate,methoxydiethylene glycol acrylate, methoxypolyethylene glycol acrylate,methoxypropylene glycol acrylate, phenoxyethyl acrylate,tetrahydrofurfuryl acrylate, isobornyl acrylate, 2-hydroxyethylacrylate, 2-hydroxypropyl acrylate, 2-hydroxy-3-phenoxypropyl acrylate,vinyl ether acrylate, vinyl ether ethoxy(meth)acrylate,2-acryloyloxyethylsuccinic acid, 2-acryloyxyethylphthalic acid,2-acryloxyethyl-2-hydroxyethyl-phthalic acid, lactone modified flexibleacrylate, and t-butylcyclohexyl acrylate, triethylene glycol diacrylate,tetraethylene glycol diacrylate, polyethylene glycol diacrylate,dipropylene glycol diacrylate, tripropylene glycol diacrylate,polypropylene glycol diacrylate, 1,4-butanediol diacrylate,1,6-hexanediol diacrylate, 1,9-nonanediol diacrylate, neopentyl glycoldiacrylate, dimethylol-tricyclodecane diacrylate, bisphenol A EO(ethylene oxide) adduct diacrylate, bisphenol A PO (propylene oxide)adduct diacrylate, hydroxypivalate neopentyl glycol diacrylate,propoxylated neopentyl glycol diacrylate, alkoxylateddimethyloltricyclodecane diacrylate and polytetramethylene glycoldiacrylate, trimethylolpropane triacrylate, EO modifiedtrimethylolpropane triacrylate, tri(propylene glycol)triacrylate,caprolactone modified trimethylolpropane triacrylate, pentaerythritoltriacrylate, pentaerithritol tetraacrylate, pentaerythritolethoxytetraacrylate, dipentaerythritol hexaacrylate, ditrimethylolpropanetetraacrylate, glycerinpropoxy triacrylate, and caprolactam modifieddipentaerythritol hexaacrylate, or an N-vinylamide such as,N-vinylcaprolactam or N-vinylformamide; or acrylamide or a substitutedacrylamide, such as acryloylmorpholine.

Other suitable monofunctional acrylates include caprolactone acrylate,cyclic trimethylolpropane formal acrylate, ethoxylated nonyl phenolacrylate, isodecyl acrylate, isooctyl acrylate, octyldecyl acrylate,alkoxylated phenol acrylate, tridecyl acrylate and alkoxylatedcyclohexanone dimethanol acrylate.

Other suitable difunctional acrylates include alkoxylated cyclohexanonedimethanol diacrylate, alkoxylated hexanediol diacrylate, dioxane glycoldiacrylate, dioxane glycol diacrylate, cyclohexanone dimethanoldiacrylate, diethylene glycol diacrylate and neopentyl glycoldiacrylate.

Other suitable trifunctional acrylates include propoxylated glycerinetriacrylate and propoxylated trimethylolpropane triacrylate.

Other higher functional acrylates include di-trimethylolpropanetetraacrylate, dipentaerythritol pentaacrylate, ethoxylatedpentaerythritol tetraacrylate, methoxylated glycol acrylates andacrylate esters.

Furthermore, methacrylates corresponding to the above-mentionedacrylates may be used with these acrylates. Of the methacrylates,methoxypolyethylene glycol methacrylate, methoxytriethylene glycolmethacrylate, hydroxyethyl methacrylate, phenoxyethyl methacrylate,cyclohexyl methacrylate, tetraethylene glycol dimethacrylate, andpolyethylene glycol dimethacrylate are preferred due to their relativelyhigh sensitivity and higher adhesion to an ink-receiver surface.

Furthermore, the inkjet inks may also contain polymerizable oligomers.Examples of these polymerizable oligomers include epoxy acrylates,aliphatic urethane acrylates, aromatic urethane acrylates, polyesteracrylates, and straight-chained acrylic oligomers.

Suitable examples of styrene compounds are styrene, p-methylstyrene,p-methoxystyrene, β-methylstyrene, p-methyl-β-methylstyrene,α-methylstyrene and p-methoxy-β-methylstyrene.

Suitable examples of vinylnaphthalene compounds are 1-vinylnaphthalene,α-methyl-1-vinylnaphthalene, β-methyl-1-vinylnaphthalene,4-methyl-1-vinylnaphthalene and 4-methoxy-1-vinylnaphthalene.

Suitable examples of N-vinyl compounds are N-vinylcarbazole,N-vinylpyrrolidone, N-vinylindole, N-vinylpyrrole, N-vinylphenothiazine,N-vinylacetoanilide, N-vinylethylacetoamide, N-vinylsuccinimide,N-vinylphthalimide, N-vinylcaprolactam and N-vinylimidazole.

The cationically polymerizable compound of the inkjet ink can be one ormore monomers, one or more oligomers or a combination thereof.

Suitable examples of cationically curable compounds can be found inAdvances in Polymer Science, 62, pages 1 to 47 (1984) by J. V. Crivello.

The cationic curable compound may contain at least one olefin,thioether, acetal, thioxane, thietane, aziridine, N-, O-, S- orP-heterocycle, aldehyde, lactam or cyclic ester group.

Examples of cationic polymerizable compounds include monomers and/oroligomers epoxides, vinyl ethers, styrenes, oxetanes, oxazolines,vinylnaphthalenes, N-vinyl heterocyclic compounds, tetrahydrofurfurylcompounds.

The cationically polymerizable monomer can be mono-, di- ormulti-functional or a mixture thereof.

Suitable cationic curable compounds having at least one epoxy group arelisted in the “Handbook of Epoxy Resins” by Lee and Neville, McGraw HillBook Company, New York (1967) and in “Epoxy Resin Technology” by P. F.Bruins, John Wiley and Sons New York (1968).

Examples of cationic curable compounds having at least one epoxy groupinclude 1,4-butanediol diglycidyl ether,3-(bis(glycidyloxymethyl)methoxy)-1,2-propane diol, limonene oxide,2-biphenyl glycidyl ether,3,4-epoxycyclohexylmethyl-3′,4′-epoxycyclohexane carboxylate,epichlorohydrin-bisphenol S based epoxides, epoxidized styrenics andmore epichlorohydrin-bisphenol F and A based epoxides and epoxidizednovolaks.

Suitable epoxy compounds including at least two epoxy groups in themolecule are alicyclic polyepoxide, polyglycidyl ester of polybasicacid, polyglycidyl ether of polyol, polyglycidyl ether ofpolyoxyalkylene glycol, polyglycidyl ester of aromatic polyol,polyglycidyl ether of aromatic polyol, urethane polyepoxy compound, andpolyepoxy polybutadiene.

Examples of cycloaliphatic bisepoxides include copolymers of epoxidesand hydroxyl components such as glycols, polyols, or vinyl ether, suchas 3,4-epoxycyclohexylmethyl-3′, 4′-epoxycyclohexylcarboxylate;bis(3,4-epoxycylohexylmethyl)adipate; limonene bisepoxide; diglycidylester of hexahydrophthalic acid.

Examples of vinyl ethers having at least one vinyl ether group includeethyl vinyl ether, n-butyl vinyl ether, isobutyl vinyl ether, octadecylvinyl ether, cyclohexyl vinyl ether, butanediol divinyl ether, hydroxylbutyl vinyl ether, cyclohexane dimethanol monovinyl ether, phenyl vinylether, p-methylphenyl vinyl ether, p-methoxyphenyl vinyl ether,α-methylphenyl vinyl ether, β-methylisobutyl vinyl ether andβ-chloroisobutyl vinyl ether, diethyleneglycol divinyl ether,triethylene glycol divinyl ether, n-propyl vinyl ether, isopropyl vinylether, dodecyl vinyl ether, diethylene glycol monovinyl ether,cyclohexanedimethanol divinyl ether, 4-(vinyloxy)butyl benzoate,bis[4-(vinyloxy)butyl]adipate, bis[4-(vinyloxy)butyl]succinate,4-(vinyloxy methyl)cyclohexylmethyl benzoate,bis[4-(vinyloxy)butyl]isophthalate,bis[4-(vinyloxymethyl)cyclohexylmethyl]glutarate,tris[4-(vinyloxy)butyl]trimellitate,4-(vinyloxy)butyl steatite,bis[4-(vinyloxy)butyl]hexanediylbiscarbamate,bis[4-(vinyloxy)methyl]cyclohexyl]methyl]terephthalate,bis[4-(vinyloxy)methyl]cyclohexyl]methyl]isophthalate,bis[4-(vinyloxy)butyl](4-methyl-1,3-phenylene)-biscarbamate,bis[4-vinyloxy)butyl](methylenedi-4,1-phenylene) biscarbamate and3-amino-1-propanol vinyl ether.

Suitable examples of oxetane compounds having at least one oxetane groupinclude 3-ethyl-3-hydroloxymethyl-1-oxetane, the oligomeric mixture1,4-bis[3-ethyl-3-oxetanyl methoxy)methyl]benzene,3-ethyl-3-phenoxymethyl-oxetane, bis([1-ethyl(3-oxetanil)]methyl)ether,3-ethyl-3-[(2-ethylhexyloxy)methyl]oxetane, 3-ethyl-[(tri-ethoxysilylpropoxy)methyl]oxetane and 3,3-dimethyl-2(p-methoxy-phenyl)-oxetane.

A preferred class of monomers and oligomers which can be used in bothradiation and cationically curable compositions are vinyl etheracrylates such as those described in U.S. Pat. No. 6,310,115 (AGFA),incorporated herein by reference. Particularly preferred compounds are2-(2-vinyloxyethoxy)ethyl(meth)acrylate, most preferably the compound is2-(2-vinyloxyethoxy)ethyl acrylate.

Initiators

The curable inkjet ink preferably also contains an initiator. Theinitiator typically initiates the polymerization reaction. The initiatorcan be a thermal initiator, but is preferably a photo-initiator. Thephoto-initiator requires less energy to activate than the monomers,oligomers and/or prepolymers to form the polymer. The photo-initiatorsuitable for use in the curable inkjet inks may be a Norrish type Iinitiator, a Norrish type II initiator or a photo-acid generator.

Thermal initiator(s) suitable for use in the curable inkjet ink includetert-amyl peroxybenzoate, 4,4-azobis(4-cyanovaleric acid),1,1′-azobis(cyclohexanecarbonitrile), 2,2′-azobisisobutyronitrile(AIBN), benzoyl peroxide, 2,2-bis(tert-butylperoxy)butane,1,1-bis(tert-butylperoxy)cyclohexane,1,1-bis(tert-butylperoxy)cyclohexane,2,5-bis(tert-butylperoxy)-2,5-dimethylhexane,2,5-bis(tert-butylperoxy)-2,5-dimethyl-3-hexyne,bis(1-(tert-butylperoxy)-1-methylethyl)benzene,1,1-bis(tert-butylperoxy)-3,3,5-trimethylcyclohexane, tert-butylhydroperoxide, tert-butyl peracetate, tert-butyl peroxide, tert-butylperoxybenzoate, tert-butylperoxy isopropyl carbonate, cumenehydroperoxide, cyclohexanone peroxide, dicumyl peroxide, lauroylperoxide, 2,4-pentanedione peroxide, peracetic acid and potassiumpersulfate.

The photo-initiator or photo-initiator system absorbs light and isresponsible for the production of initiating species, such as freeradicals and cations. Free radicals and cations are high-energy speciesthat induce polymerization of monomers, oligomers and polymers and withpolyfunctional monomers and oligomers thereby also inducingcross-linking.

Irradiation with actinic radiation may be realized in two steps bychanging wavelength or intensity. In such cases it is preferred to use 2types of photo-initiator together.

A combination of different types of initiator, for example, aphoto-initiator and a thermal initiator can also be used.

A preferred Norrish type I-initiator is selected from the groupconsisting of benzoinethers, benzil ketals, α,α-dialkoxyacetophenones,α-hydroxyalkylphenones, α-aminoalkylphenones, acylphosphine oxides,acylphosphine sulphides, α-haloketones, α-halosulfones andα-halophenylglyoxalates.

A preferred Norrish type II-initiator is selected from the groupconsisting of benzophenones, thioxanthones, 1,2-diketones andanthraquinones. A preferred co-initiator is selected from the groupconsisting of an aliphatic amine, an aromatic amine and a thiol.Tertiary amines, heterocyclic thiols and 4-dialkylamino-benzoic acid areparticularly preferred as co-initiator.

Suitable photo-initiators are disclosed in CRIVELLO, J. V., et al.VOLUME III: Photoinitiators for Free Radical Cationic. 2nd edition.Edited by BRADLEY, G. London, UK: John Wiley and Sons Ltd, 1998. p.287-294.

Specific examples of photo-initiators may include, but are not limitedto, the following compounds or combinations thereof: benzophenone andsubstituted benzophenones, 1-hydroxycyclohexyl phenyl ketone,thioxanthones such as isopropylthioxanthone,2-hydroxy-2-methyl-1-phenylpropan-1-one,2-benzyl-2-dimethylamino-(4-morpholinophenyl)butan-1-one, benzildimethylketal, bis(2,6-dimethylbenzoyl)-2,4,4-trimethylpentylphosphineoxide, 2,4,6trimethylbenzoyldiphenylphosphine oxide,2-methyl-1-[4-(methylthio)phenyl]-2-morpholinopropan-1-one,2,2-dimethoxy-1, 2-diphenylethan-1-one or5,7-diiodo-3-butoxy-6-fluorone, diphenyliodonium fluoride andtriphenylsulfonium hexafluophosphate.

Suitable commercial photo-initiators include IRGACURE™ 184, IRGACURE™500, IRGACURE™ 907, IRGACURE™ 369, IRGACURE™ 1700, IRGACURE™ 651,IRGACURE™ 819, IRGACURE™ 1000, IRGACURE™ 1300, IRGACURE™ 1870, DAROCUR™1173, DAROCUR™ 2959, DAROCUR™ 4265 and DAROCUR™ ITX available from CIBASPECIALTY CHEMICALS, Lucerin TPO available from BASF AG, ESACURE™ KT046,ESACURE™ KIP150, ESACURE™ KT37 and Esacure™ EDB available from LAMBERTI,H-NU™ 470 and H-NU™ 470× available from SPECTRA GROUP Ltd.

Suitable cationic photo-initiators include compounds, which form aproticacids or Bronstead acids upon exposure to ultraviolet and/or visiblelight sufficient to initiate polymerization. The photo-initiator usedmay be a single compound, a mixture of two or more active compounds, ora combination of two or more different compounds, i.e. co-initiators.Non-limiting examples of suitable cationic photo-initiators arearyldiazonium salts, diaryliodonium salts, triarylsulphonium salts,triarylselenonium salts and the like.

Suitable commercial cationic photoinitiators include R-GEN™ 1130, R-GEN™BF-1172, R-GEN™ 261, CHIVACURE™ 1176 and Chivacure™ 1190 from ChitecTechnology Co., Ltd.; IRGACURE™ 250 from Ciba Specialty Products;UV9387C and UV9380C from GE Silicones; CYRACURE™ Photoinitiator UVI-6976and UVI-6992 from The Dow Chemical Company; OMNICAT™ series from IGMResins, including OMNICAT 432™ (sulfonium type), OMNICAT™ 440 (iodoniumtype), OMNICAT™ 445 (iodonium type), OMNICAT™ 550 and OMNICAT™ 650(polymeric type); ESACURE™ 1064, ESACURE™ 1187 and ESACURE™ 1188 fromLamberti S.p.A.; Adeka OPTOMER™ SP series of aromatic sulfonium typescationic photo-initiatiors from Adeka Corporation, e.g. Adeka OPTOMER™SP-152; and OMPH076 from ABCR Gmbh & Co. KG, a blend of an aromaticsulfonium and aromatic thioether (available by B & S Specialties BVunder the tradename Sarcat KI85).

The curable inkjet ink may contain a photo-initiator system containingone or more photo-initiators and one or more sensitizers that transferenergy to the photo-initiator(s). Suitable sensitizers includephotoreducible xanthene, fluorene, benzoxanthene, benzothioxanthene,thiazine, oxazine, coumarin, pyronine, porphyrin, acridine, azo, diazo,cyanine, merocyanine, diarylmethyl, triarylmethyl, anthraquinone,phenylenediamine, benzimidazole, fluorochrome, quinoline, tetrazole,naphthol, benzidine, rhodamine, indigo and/or indanthrene dyes. Theamount of the sensitizer is in general from 0.01 to 15 wt %, preferablyfrom 0.05 to 5 wt %, based in each case on the total weight of thecurable inkjet ink.

In order to increase the photosensitivity further, the curable inkjetink may additionally contain co-initiators. For example, the combinationof titanocenes and trichloromethyl-s-triazines, of titanocenes andketoxime ethers and of acridines and trichloromethyl-s-triazines isknown. A further increase in sensitivity can be achieved by addingdibenzalacetone or amino acid derivatives. The amount of co-initiator orco-initiators is in general from 0.01 to 20 wt %, preferably from 0.05to 10 wt %, based in each case on the total weight of the curable inkjetink.

Suitable examples of co-initiators can be categorized in 4 groups:

-   (1) tertiary aliphatic amines such as methyldiethanolamine,    dimethylethanolamine, triethanolamine, triethylamine and    N-methylmorpholine;-   (2) aromatic amines such as amylparadimethylaminobenzoate,    2-n-butoxyethyl-4-(dimethylamino)benzoate,    2-(dimethylamino)ethylbenzoate, ethyl-4-(dimethylamino)benzoate, and    2-ethylhexyl-4-(dimethylamino)benzoate;-   (3) (meth)acrylated amines such as dialkylamino alkyl(meth)acrylates    (e.g., diethylaminoethylacrylate) or    N-morpholinoalkyl-(meth)acrylates (e.g.,    N-morpholinoethyl-acrylate); and-   (4) amides or ureas.    The preferred co-initiators are aminobenzoates.

A preferred initiator system is2,2′-bis(o-chlorophenyl)-4,4′,5,5′-tetraphenyl-(7CI,8CI)4,4′-Bi-4H-imidazole corresponding to the chemical formula:

in the presence of a co-initiator such as 2-mercapto benzoxazole.

Another preferred type of initiator is an oxime ester. A suitableexample has as chemical formula:

A preferred amount of initiator is 0.3-50 wt % of the total weight ofthe curable liquid, and more preferably 1-15 wt % of the total weight ofthe curable inkjet ink.

Irradiation with actinic radiation may be realized in two steps bychanging wavelength or intensity. In such cases it is preferred to use 2types of photo-initiator together.

Inhibitors

Suitable polymerization inhibitors include phenol type antioxidants,hindered amine light stabilizers, phosphor type antioxidants,hydroquinone monomethyl ether commonly used in (meth)acrylate monomers,and hydroquinone, t-butylcatechol, pyrogallol may also be used.

Suitable commercial inhibitors are, for example, SUMILIZER™ GA-80,SUMILIZER™ GM and SUMILIZER™ GS produced by Sumitomo Chemical Co. Ltd.;GENORAD™ 16, GENORAD™ 18 and GENORAD™ 20 from Rahn AG; IRGASTAB™ UV10and IRGASTAB™ UV22, TINUVIN™ 460 and CGS20 from Ciba SpecialtyChemicals; FLOORSTAB™ UV range (UV-1, UV-2, UV-5 and UV-8) fromKromachem Ltd, ADDITOL™ S range (S100, S110, S120 and S130) from CytecSurface Specialties.

However, most preferably the inhibitor is a polymerizable inhibitor.

Since excessive addition of these polymerization inhibitors will lowerthe ink sensitivity to curing, it is preferred that the amount capableof preventing polymerization is determined prior to blending. The amountof a polymerization inhibitor is preferably lower than 2 wt % of thetotal ink.

Surfactants

The surfactant(s) can be anionic, cationic, non-ionic, or zwitter-ionicand are usually added in a total quantity less than 20 wt % based on thetotal weight of the inkjet ink and particularly in a total less than 10wt % based on the total weight of the inkjet ink.

Suitable surfactants include fluorinated surfactants, fatty acid salts,ester salts of a higher alcohol, alkylbenzene sulphonate salts,sulphosuccinate ester salts and phosphate ester salts of a higheralcohol (for example, sodium dodecylbenzenesulphonate and sodiumdioctylsulphosuccinate), ethylene oxide adducts of a higher alcohol,ethylene oxide adducts of an alkylphenol, ethylene oxide adducts of apolyhydric alcohol fatty acid ester, and acetylene glycol and ethyleneoxide adducts thereof (for example, polyoxyethylene nonylphenyl ether,and SURFYNOL™ 104, 104H, 440, 465 and TG available from AIR PRODUCTS &CHEMICALS INC.).

For non-aqueous inkjet inks preferred surfactants are selected fromfluoro surfactants (such as fluorinated hydrocarbons) and siliconesurfactants. The silicones are typically siloxanes and can bealkoxylated, polyether modified, polyether modified hydroxy functional,amine modified, epoxy modified and other modifications or combinationsthereof. Preferred siloxanes are polymeric, for examplepolydimethylsiloxanes.

In radiation curable inkjet inks a fluorinated or silicone compound maybe used as a surfactant, however, a cross-linkable surfactant would bepreferred. It is therefore preferred to use a copolymerizable monomerhaving surface-active effects, for example, polyacrylate copolymers,silicone modified acrylates, silicone modified methacrylates, acrylatedsiloxanes, polyether modified acrylic modified siloxanes, fluorinatedacrylates, and fluorinated methacrylates; these acrylates can be mono-,di-, tri- or higher functional (meth)acrylates.

Surfactants are known for use in inkjet inks to reduce the surfacetension of the ink and to reduce the contact angle on the substrate,i.e. to improve the wetting of the substrate by the ink. On the otherhand, the jettable fluid must meet stringent performance criteria inorder to be adequately jettable with high precision, reliability andduring an extended period of time. To achieve both wetting of thesubstrate by the ink and high jetting performance, typically, thesurface tension of the ink is reduced by the addition of one or moresurfactants. In the case of curable inkjet inks, however, the surfacetension of the inkjet ink is not only determined by the amount and typeof surfactant, but also by the polymerizable compounds, the polymericdispersants and other additives in the ink composition.

Depending upon the application a surfactant can be used with a high, lowor intermediate dynamic surface tension. Silicone surfactants aregenerally known to have low dynamic surface tensions while fluorinatedsurfactants are known to have higher dynamic surface tensions.

Useful commercially available fluorinated surfactants are for examplethe ZONYL™ range of fluoro-surfactants from DUPONT and the FLUORAD™range of fluoro-surfactants from 3M. Other fluorinated surfactants aree.g. described in EP 1412438 A (3M).

Silicone surfactants are often preferred in curable inkjet inks,especially the reactive silicone surfactants, which are able to bepolymerized together with the polymerizable compounds during the curingstep.

Useful commercially available silicone surfactants are oftenpolysiloxane surfactants, especially polyether modified polysiloxanes,preferably with one or more acrylate function in order to becomepolymerizable.

Examples of useful commercial silicone surfactants are those supplied byBYK CHEMIE GMBH (including BYK™-302, 307, 310, 331, 333, 341, 345, 346,347, 348, UV3500, UV3510 and UV3530), those supplied by TEGO CHEMIESERVICE (including TEGO RAD™ 2100, 2200N, 2250, 2300,2500, 2600 and2700), EBECRYL™ 1360 a polysilixone hexaacrylate from CYTEC INDUSTRIESBV and EFKA™-3000 series (including EFKA™-3232 and EFKA™-3883) from EFKACHEMICALS B.V.

Binders

The inkjet inks of the inkjet ink set according to a preferredembodiment of the present invention may include a binder resin. Thebinder functions as a viscosity controlling agent and also providesfixability relative to a substrate, e.g. a polyvinyl chloride substrate.The binder preferably has a good solubility in the solvent(s).

Non-aqueous inkjet ink compositions preferably include a binder resin.The binder functions as a viscosity controlling agent and also providesfixability relative to the polymeric resin substrate, e.g. a polyvinylchloride substrate, also called vinyl substrate. The binder must beselected to have a good solubility in the solvent(s).

Suitable examples of binder resins include acrylic resins, modifiedacrylic resins, styrene acrylic resins, acrylic copolymers, acrylateresins, aldehyde resins, rosins, rosin esters, modified rosins andmodified rosin resins, acetyl polymers, acetal resins such as polyvinylbutyral, ketone resins, phenolic resins and modified phenolic resins,maleic resins and modified maleic resins, terpene resins, polyesterresins, polyamide resins, polyurethane resins, epoxy resins, vinylresins, vinyl chloride-vinyl acetate copolymer resins, cellulose typeresins such as nitro cellulose, cellulose acetopropionate and celluloseacetate butyrate, and vinyl toluene-α-methylstylene copolymer resin.These binders may be used alone or in a mixture thereof. The binder ispreferably a film-forming thermoplastic resin.

The amount of binder resin in inkjet ink is preferably in the range of0.1 to 30 wt %, more preferably 1 to 20 wt %, most preferably 2 to 10 wt% based on the total weight of the inkjet ink.

Humectants

If the inkjet inks contain organic solvents or water, preferably atleast one humectant is present in the inks to prevent the clogging ofthe nozzle, due to its ability to slow down the evaporation rate of ink.

Suitable humectants include triacetin, N-methyl-2-pyrrolidone, glycerol,urea, thiourea, ethylene urea, alkyl urea, alkyl thiourea, dialkyl ureaand dialkyl thiourea, diols, including ethanediols, propanediols,propanetriols, butanediols, pentanediols, and hexanediols; glycols,including propylene glycol, polypropylene glycol, ethylene glycol,polyethylene glycol, diethylene glycol, tetraethylene glycol, andmixtures and derivatives thereof. Preferred humectants are triethyleneglycol mono butylether, glycerol and 1,2-hexanediol. The humectant ispreferably added to the inkjet ink formulation in an amount of 0.1 to 40wt % of the formulation, more preferably 0.1 to 10 wt % of theformulation, and most preferably approximately 4.0 to 6.0 wt %.

Other Additives

The inkjet inks of the inkjet ink set according to a preferredembodiment of the present invention may include other additives such asbuffering agents, anti-mold agents, pH adjustment agents, electricconductivity adjustment agents, chelating agents, anti-rusting agents,light stabilizers, dendrimers, polymers, cross-linking agents, solubleelectrolytes as conductivity aid, sequestering agents and chelatingagents, compounds to introduce additional security features and thelike. Such additives may be included in the colour inkjet inks of theinkjet ink set according to a preferred embodiment of the presentinvention in any effective amount, as desired.

Compounds to introduce additional security features include afluorescent compound, a phosphorescent compound, a thermochromiccompound, an iridescent compound and a magnetic particle. SuitableUV-fluorescent and phosphorescent compounds include LUMILUX™ luminescentpigments from HONEYWELL, UVITEX™ OB from CIBA-GEIGY, KEYFLUOR™ dyes andpigments from KEYSTONE and fluorescent dyes from SYNTHEGEN.

The colour inkjet inks of the inkjet ink set according to a preferredembodiment of the present invention may further include conducting orsemi-conducting polymers, such as polyanilines, polypyrroles,polythiophenes such as poly(ethylenedioxythiophene) (PEDOT), substitutedor unsubstituted poly(phenylenevinylenes) (PPV's) such as PPV andMEH-PPV, polyfluorenes such as PF6, etc.

Preparation of Pigmented Inkjet Inks

The average particle size and distribution is an important feature forinkjet inks. The inkjet ink may be prepared by precipitating or millingthe pigment in the dispersion medium in the presence of the dispersant.

Mixing apparatuses may include a pressure kneader, an open kneader, aplanetary mixer, a dissolver, and a Dalton Universal Mixer. Suitablemilling and dispersion apparatuses are a ball mill, a pearl mill, acolloid mill, a high-speed disperser, double rollers, a bead mill, apaint conditioner, and triple rollers. The dispersions may also beprepared using ultrasonic energy.

Many different types of materials may be used as milling media, such asglasses, ceramics, metals, and plastics. In a preferred embodiment, thegrinding media can include particles, preferably substantially sphericalin shape, e.g. beads consisting essentially of a polymeric resin oryttrium stabilized zirconium oxide beads.

In the process of mixing, milling and dispersion, each process isperformed with cooling to prevent build up of heat, and for radiationcurable inkjet inks as much as possible under light conditions in whichactinic radiation has been substantially excluded.

The inkjet ink may contain more than one pigment, the inkjet ink may beprepared using separate dispersions for each pigment, or alternativelyseveral pigments may be mixed and co-milled in preparing the dispersion.

The dispersion process can be carried out in a continuous, batch orsemi-batch mode.

The preferred amounts and ratios of the ingredients of the mill grindwill vary widely depending upon the specific materials and the intendedapplications. The contents of the milling mixture include the mill grindand the milling media. The mill grind includes pigment, polymericdispersant and a liquid carrier. For inkjet inks, the pigment is usuallypresent in the mill grind at 1 to 50 wt %, excluding the milling media.The weight ratio of pigment over polymeric dispersant is 20:1 to 1:2.

The milling time can vary widely and depends upon the pigment, selectedmechanical device and residence conditions, the initial and desiredfinal particle size, etc. In a preferred embodiment of the presentinvention pigment dispersions with an average particle size of less than100 nm may be prepared.

After milling is completed, the milling media is separated from themilled particulate product (in either a dry or liquid dispersion form)using conventional separation techniques, such as by filtration, sievingthrough a mesh screen, and the like. Often the sieve is built into themill, e.g. for a bead mill. The milled pigment concentrate is preferablyseparated from the milling media by filtration.

In general it is desirable to make the inkjet inks in the form of aconcentrated mill grind, which is subsequently diluted to theappropriate concentration for use in the inkjet printing system. Thistechnique permits preparation of a greater quantity of pigmented inkfrom the equipment. By dilution, the inkjet ink is adjusted to thedesired viscosity, surface tension, colour, hue, saturation density, andprint area coverage for the particular application.

Examples Materials

All materials used in the following examples were readily available fromstandard sources such as Aldrich Chemical Co. (Belgium) and Acros(Belgium) unless otherwise specified.

-   PV19 is an abbreviation used for HOSTAPERM™ Red E5B 02, a C.I.    Pigment violet 19 from CLARIANT.-   PB15:3 (A) is an abbreviation used for Hostaperm Blue B4G, a C.I.    Pigment Blue 15:3 pigment from CLARIANT.-   PB15:3 (B) is an abbreviation used for SUNFAST™ Blue 249-1284, a    C.I. Pigment Blue 15:3 pigment from SUN CHEMICAL.-   PY150 is an abbreviation used for CHROMOPHTAL™ Yellow LA, a C.I.    Pigment Yellow 150 from CIBA SPECIALTY CHEMICALS.-   PBlack7 is SPECIAL BLACK™ 550, a carbon black available from DEGUSSA-   TR52 is TIOXIDE TR 52™, a surface modified titanium dioxide from    HUNTSMAN CHEMICAL GROUP.-   SOLSPERSE™ 35000, a polyethyleneimine-polyester hyperdispersant from    NOVEON.-   S39000 is an abbreviation for SOLSPERSE™ 39000, a    polyethyleneimine-polyester hyperdispersant from NOVEON.-   S5000 is an abbreviation for SOLSPERSE™ 5000 is a sulfonated copper    phthalocyanine dispersion synergist from NOVEON.-   SOLSPERSE™36000 is a polymeric dispersant for TiO₂ from NOVEON.-   Genorad™ 16 is a polymerization inhibitor from RAHN AG.-   DPGDA is dipropyleneglycoldiacrylate from SARTOMER.-   SR9003 is an abbreviation for SARTOMER™ SR9003, a propoxylated    neopentyl glycol diacrylate monomer available from SARTOMER.-   VEEA is 2-(2-vinyloxyethoxy)ethyl acrylate from NIPPON SHOKUBAI.-   IBOA is isobornylacrylate available as Sartomer 506D from SARTOMER.-   TMPTA is Trimethylolpropane Triacrylate available as SR351 from    SARTOMER.-   ESACURE™ KTO46 is a mixture of trimethylbenzoyldiphenylphosphine    oxide, alfa-hydroxyketone en benzophenone derivatives avaialble from    FRATELLI LAMBERTI SPA.-   DAROCUR™ TPO is 2,4,6-trimethylbenzoyl-diphenyl-phosphineoxide from    CIBA SPECIALTY CHEMICALS.-   GENOCURE™ EPD is ethyl 4-dimethylaminobenzoate from RAHN AG.-   GENOCURE™ TPO is 2,4,6-trimethylbenzoyl-diphenyl-phosphineoxide from    RAHN AG.-   DAROCUR™ ITX is an isomeric mixture of 2- and    4-isopropylthioxanthone from CIBA SPECIALTY CHEMICALS.-   EBECRYL™ 350 is a polysilioxane diacrylate from CYTEC INDUSTRIES BV.-   EBECRYL™ 1360 is a polysiloxane hexaacrylate from CYTEC INDUSTRIES    BV.-   EBESOL is a 30% solution of EBECRYL™ 1360 in VEEA.-   TEGOGLIDE™ 410 is a polysiloxan polyether copolymer from    GOLDSCHMIDT.-   TEGOSOL is a 30% solution of TEGOGLIDE™ 410 in IBOA.-   GENVEEA is a 50% solution of GENORAD™ 16 in VEEA.-   GENIBOA is a 50% solution of GENORAD™ 16 in IBOA.-   Black PET is an AGFA IDEALINE™ Silver Film RPF (Red Phototooling    Film) which was exposed and developed to maximum density of 4.2 to    4.5 as measured with a MACBETH™ TD908 without any filter.

Measurement Methods 1. Measurement of TURBISCAN™ Stability Index

A vial with a sample of 20 mL of ink was measured at 20° C. using asedimentometer TURBISCAN™ LabExpert in combination with TURBISCAN™Easysoft (manufactured by FORMULACTION, France) that can evaluate thesettling properties from intensity distributions of back scattered lightand transmitted light in the height direction of a sample.Backscattering and transmission were measured at 880 nm at the top 30 mmof the vial during 13 days. A TURBISCAN™ Stability Index is calculatedwherein a higher value corresponds with a less stable ink.

2. Average Particle Size of Concentrated Pigment Dispersion (Malvern)

The average particle size of pigment particles in concentrated pigmentdispersions was determined by photon correlation spectroscopy at awavelength of 633 nm with a 4 mW HeNe laser on a diluted sample of thepigmented inkjet ink. The particle size analyzer used was a MALVERN™nano-S available from Goffin-Meyvis.

The sample was prepared by addition of one drop of ink to a cuvetcontaining 1.5 mL ethyl acetate and mixed until a homogenous sample wasobtained. The measured particle size is the average value of 3consecutive measurements consisting of 6 runs of 20 seconds.

3. Average Particle Size of Inkjet Ink (BI90)

The average particle size of pigment particles in a non-aqueous inkjetink was determined with a Brookhaven Instruments Particle Sizer BI90plusbased upon the principle of dynamic light scattering. The ink ordispersion was diluted with ethyl acetate to a pigment concentration of0.002 wt %. The measurement settings of the BI90plus were: 5 runs at 23°C., angle of 90°, wavelength of 635 nm and graphics=correction function.

For good inkjet characteristics (jetting characteristics and printquality) the average particle size of the dispersed particles in acolour inkjet ink should be less than 200 nm, preferably less than 150nm.

4. Viscosity

The viscosity of the white inkjet inks was measured using a BrookfieldDV-II+ viscometer at 40° C. at 12 rotations per minute (RPM) using a CPE40 spindle, which measurement corresponds to a viscosity measured at 40°C. at a shear rate of 90 s⁻¹.

5. Surface Energy of the Substrates

The Owens-Wendt equation was used for calculating the surface energy ofa substrate σ_(s) in the same manner as disclosed in US 2005190245(AGFA).

6. Surface Tension of Inks

The surface tension of the inkjet inks was measured with a KRÜSStensiometer K9 at 25° C. after 60 seconds.

7. Measurement of CieL*a*b* Parameters

Printed samples were measured with a spectrophotometer (Gretag SPM50) todetermine the coordinates of the L*a*b* colours system of the colourdifference indication method specified in CIE (Commission Internationalde l'Eclairage). In this case, the measurement was carried out underconditions of light source D50, provision of no light source filter,absolute white as reference white, and angle of visibility 2°.

Example 1

This example illustrates how sedimentation problems in inkjet printingcan be strongly reduced by using higher concentrations of TiO₂ in awhite inkjet ink and by adjusting the temperature of the white ink infunction of its demand for printing.

Preparation of White Pigment Dispersion

Concentrated white pigment dispersions W1 and W2 were prepared accordingto Table 3.

TABLE 3 wt % of: W1 W2 TR52 50.0 50.0 SOLSPERSE ™ 10.0 10.0 36000GENORAD ™ 16 1.0 1.0 VEEA 39.0 — IBOA — 39.0

The concentrated pigment dispersion W1 was made by mixing 500.0 g of thewhite pigment TR52, 20.0 g of a 50% solution of the inhibitor GENORAD™16 in VEEA and 333.3 g of a 30% solution of the polymeric dispersantSolsperse™ 36000 in VEEA for 30 minutes using a DISPERLUX™ LaboratoryDissolver YELLOW075 from DISPERLUX S.A.R.L., Luxembourg. This mixturewas subsequently milled in an Eiger Lab Bead mill (from EIGER TORRANCELtd.) using yttrium-stabilized zirconium oxide-beads of 1-1.6 mmdiameter (“high wear resistant zirconia grinding media” from TOSOH Co.).The bead mill is filled for 52% with the grinding beads and water-cooledduring milling at 1500 rpm for 5 minutes. The concentrated pigmentdispersion W1 had an average particles size of 317 nm measured with aMALVERN™ nano-S particle size analyzer.

The concentrated pigment dispersion W2 was made by mixing 500.0 g of thewhite pigment TR52, 20.0 g of a 50% solution of the inhibitor GENORAD™16 in IBOA and 333.3 g of a 30% solution of the polymeric dispersantSOLSPERSE™ 36000 in IBOA for 30 minutes using a KOTHOFF MS1 dispersionsystem of Hans Kothoff, Apparate- and Maschinenbau, Rodenkirchen,Germany. The concentrated pigment dispersion W1 had an average particlessize of 335 nm measured with a MALVERN™ nano-S particle size analyzer.

Preparation of White Inkjet Inks

Curable white inkjet inks INK-1 to INK-6 were prepared from theconcentrated pigment dispersion W1 and W2 by adding the remainingcomponents under stirring at 20° C. to obtain a composition as shown inTable 4.

TABLE 4 wt % of INK-1 INK-2 INK-3 INK-4 INK-5 INK-6 W1 34.0 51.0 68.0 —— — W2 — — — 34.0 51.0 68.0 IBOA — — — 53.7 36.7 19.7 VEEA 26.9 13.9 0.8— — — TMPTA 26.8 22.8 18.9 — — — ESACURE ™ 10.0 10.0 10.0 10.0 10.0 10.0KTO46 EBESOL 1.0 1.0 1.0 — — — TEGOSOL — — — 1.0 1.0 1.0 GENVEEA 1.3 1.31.3 — — — GENIBOA — — — 1.3 1.3 1.3

Evaluation

The viscosity of the inkjet inks INK-1 to INK-6 was measured at 40° C.using a Brookfield DV-II+ viscometer and is listed in Table 5. It can beobserved that the viscosity increases with an increasing content ofTiO₂.

TABLE 5 Viscosity Ink wt % of TiO₂ (mPa · s) INK-1 17 11 INK-2 26 16INK-3 34 20 INK-4 17 11 INK-5 26 16 INK-6 34 24

The viscosity of the inks INK-2, INK-3, INK-5 and INK-6 can be reducedby increasing the temperature of the ink.

TABLE 6 Viscosity (mPa · s) Temperature INK-2 INK-3 INK-5 INK-6 40° C.16 20 16 24 50° C. 12 — 12 — 55° C. 11 15 11 — 60° C. — 11 — 14 65° C. —— — 12

The sedimentometer TURBISCAN™ LabExpert was used to determine thesedimentation speed. From Table 7, it is clear that the moreconcentrated inks INK-2, INK-3, INK-5 and INK-6 exhibited a smallervalue for the Turbiscan Stability Index and hence were thus much morestable.

TABLE 7 Turbiscan Stability Ink index INK-1 13 INK-2 5 INK-3 6 INK-4 19INK-5 10 INK-6 10

Preparation and Coating of White Ink-Mixtures

The inkjet inks INK-2 and INK-3, respectively inkjet inks INK-5 andINK-6, were diluted with monomers, surfactants, photoinitiators andstabilizer in order to obtain the same composition as inkjet ink INK-1,respectively inkjet ink INK-4, thus allowing a fair comparison betweenthe inks at an equal content of 17 wt % of TiO2 based upon the totalweight of the ink.

For simulating sedimentation conditions in a printer, the whiteink-mixtures mentioned above were prepared after 1 hour, 6 hours, 24hours, 1 week, 2 weeks and 9 weeks of unstirred storage at roomtemperature, by taking samples with a pipette at the top of thecontainers of the inks INK-2 and INK-3 and INK-5 and INK-6 andsubsequently diluting them with the other components under stirring themfor 5 minutes. Samples of INK-1 and INK-4 were also taken with a pipetteat the top of their container after 1 hour, 6 hours, 24 hours, 1 week, 2weeks and 9 weeks of unstirred storage at room temperature, and thenstirred for 5 minutes before testing.

The inkjet inks INK-1, INK-4 and the white ink-mixtures were each coatedon a substrate Black PET using a bar coater and a 10 μm wired bar. Eachcoated layer was cured using a Fusion DRSE-120 conveyer, equipped with aFusion VPS/I600 lamp (D-bulb), which transported the samples under theUV lamp on a conveyer belt at a speed of 20 m/min.

The lightness values of the coated white layers were determined and arelisted in Table 8.

TABLE 8 Lightness value L for white ink: After storage Diluted DilutedDiluted Diluted at 20° C. for: INK-1 INK-2 INK-3 INK-4 INK-5 INK-6 1hour 67 67 67 63 68 68 6 hours 65 68 69 63 67 66 24 hours 66 68 69 63 6665 1 week 45 60 67 57 64 69 2 weeks 12 24 64 47 52 66 9 weeks 10 31 5713 47 52

Table 8 illustrates that the white ink-mixtures diluted after more thanone week of storage in an inkjet printer from the higher concentratedinks INK-2 and INK-3, respectively INK-5 and INK-6, still exhibited goodimage quality, while the inkjet inks INK-1 and INK-4 were alreadydeteriorating rapidly.

The undiluted inkjet inks INK-2, INK-3, INK-5 and INK-6 were coated andcured in the same manner as above, and compared with inkjet inks INK-1and INK-4. The CieL*a*b* parameters were determined for all coated inksINK-1 to INK-6, as well as for the uncoated Black PET substrate (Noink). Another advantage that became apparent from the Cielabmeasurements of Table 9, is that for reaching a certain level of opacityless white ink is needed. Thinner layers are not only economicallyinteresting, but exhibit different physical properties, such as higherflexibility.

TABLE 9 CieL*a*b* No parameter ink INK-1 INK-2 INK-3 INK-4 INK-5 INK-6L* 9 68 75 77 63 72 74 a* 1.7 −3.1 −3.0 −3.0 −3.6 −3.3 −3.1 b* 1.2 −5.4−5.2 −5.4 −7.4 −6.0 −5.6

Example 2

This example shows an inkjet ink set which has been optimized forjetting with Nova 256/80 AAA printheads (available from FUJIFILMDIMATIX) at a jetting temperature of 40° C.

Printheads in inkjet printers are normally all operated at the sametemperature, which simplifies not only the requirements for atemperature control system, but also results in a better image qualityin radiation curable inkjet printing since the curing speed isinfluenced by differences in ink temperature.

The multi-density inkjet ink set consists of a neutral black ink (K), amagenta (M) and a light magenta (LM) ink, a cyan (C) and a light cyan(LC) ink and a yellow ink (Y) composed as shown in Table 10 and thewhite inkjet ink INK-3 (composition see Example 1).

TABLE 10 in wt % of ink K M LM C LC Y DPGDA 60.92 60.80 32.94 23.7933.36 41.20 SR9003 20.00 20.00 50.00 60.00 50.00 41.40 PBlack7 1.54 — —— — — PV19 0.55 3.60 0.51 — — — PB15:3 (A) 0.55 — — — — — PB15:3 (B) — —— 1.17 0.17 — PY150 — — — — — 1.80 S39000 2.64 1.80 0.25 1.17 0.17 1.80S 5000 — — — 0.06 0.01 — DAROCUR ™ ITX 5.00 5.00 — 5.00 — 5.00 DAROCUR ™TPO — — 10.00 — 10.00 — GENOCURE ™ EPD 7.50 7.50 5.00 7.50 5.00 7.50EBECRYL ™ 350 0.30 0.30 0.30 0.30 0.30 0.30 GENORAD ™ 16 1.00 1.00 1.001.00 1.00 1.00 viscosity at 11 12 11 10 10 10 40° C. (mPa · s) surfacetension 24.2 24.0 24.0 24.5 24.2 24.1 at 25° C. (mN/m)

The dispersion quality of colour pigments in inkjet inks tend todeteriorate when the ink is kept at higher temperatures for a longperiod. The average particle size of the inks in the inkjet ink set ofTable 10 was determined after preparation and again after a heattreatment of one week at 80° C. The results are given in Table 11.

TABLE 11 Measured Average particle size in nm (BI90) after K M LM C LC Ypreparation 134 140 134 103 103 202 1 week 230 291 209 112 109 204 at80° C.

From Table 11, it is clear that particularly the dispersion stability ofthe black and the magenta inkjet inks of this ink set suffer fromprolonged storage at higher temperature. Therefore, these colour inksare preferably used with printheads operating at a lower temperature,e.g. 40° C. In order to obtain reduced sedimentation problems with thewhite inkjet ink, it becomes necessary to jet the white inkjet ink at ahigher temperature of about 60° C. (see Table 6) because viscosity playsan important role in the jetting performance of an inkjet ink.

While preferred embodiments of the present invention have been describedabove, it is to be understood that variations and modifications will beapparent to those skilled in the art without departing the scope andspirit of the present invention. The scope of the present invention,therefore, is to be determined solely by the following claims.

1-14. (canceled)
 15. An inkjet printing method comprising the steps of:providing a white inkjet ink and at least one color inkjet ink to aninkjet printer; and jetting the white inkjet ink at a temperature higherthan a temperature of the at least one color inkjet ink onto anink-receiver; wherein a difference in jetting temperature between thewhite inkjet ink and the at least one color inkjet ink is at least 5° C.16. The inkjet printing method according to claim 15, further comprisingthe step of reducing the temperature of the white ink in a printhead byat least 5° C. when printing images not requiring the white inkjet inkor when the inkjet printer is not used for more than 24 hours.
 17. Theinkjet printing method according to claim 16, wherein a temperature of aprinthead including the at least one color inkjet ink remains at itsjetting temperature.
 18. The inkjet printing method according to claim15, further comprising the step of surface printing or backing printingof the white inkjet ink on a transparent or translucent substrate inorder to form a reflection image.
 19. The inkjet printing methodaccording to claim 15, wherein the at least one color inkjet ink isjetted on a partially cured layer of the white inkjet ink.
 20. Theinkjet printing method according to claim 15, wherein the white inkjetink includes a pigment with a refractive index greater than 1.60. 21.The inkjet printing method according to claim 15, wherein the pigmentwith a refractive index greater than 1.60 is titanium dioxide.
 22. Theinkjet printing method according to claim 21, wherein an averageparticle size of the titanium dioxide measured with photon correlationspectroscopy at a wavelength of 633 nm is between 0.2 μm and 0.5 μm. 23.The inkjet printing method according to claim 15, wherein a viscosity ofthe white inkjet ink is at least 4 mPa·s greater than a viscosity of theat least one color inkjet ink when the viscosity is measured at 40° C.with a Brookfield DV-II+Pro at 12 rotations per minute.
 24. The inkjetprinting method according to claim 15, wherein the white inkjet inkincludes at least 26 wt % of a white pigment based upon a total weightof the white inkjet ink.
 25. The inkjet printing method according toclaim 15, further comprising the step of curing the white inkjet ink byheat, radiation, and/or electron beam.
 26. The inkjet printing methodaccording to claim 15, wherein the white inkjet ink includes apolymerizable compound selected from the group consisting ofisobornylacrylate, phenoxyethyl acrylate, tetrahydrofurfurylacrylate,2-(2-vinyloxyethoxy)ethyl(meth)acrylate, andN-vinylcaprolactam.